A Few Heads Taller Than the Tallest?
Assuming that the rest of the Greek deities were about the same size as Aphrodite, then the Homeric Hymn to Aphrodite strongly suggests that they were something like abnormally large or tall humans, or, at the very least, as tall as the house of a shepherd-prince in Dardania a generation before the Trojan War.
The hymn tells the story of the conception of Aineias [Aeneas], the Dardanian prince who later escaped the destruction of Troy to eventually settle in Italy, and how his parents—the mortal prince Ankhises [Anchises] and the goddess Aphrodite—met when, inflamed with desire for Ankhises, Aphrodite came to his homestead on the slopes of Mt Ida in Troas.
Even though the hymn is sure to point out that Ankhises is himself as shapely and attractive as one of the gods, nonetheless Aphrodite takes special care (in Line 82) to take the form of "an unwed maiden in size of length and mien" because, apparently, her real height would freak him out.
But even so, as soon as Ankhises lays eyes upon her, he is so bedazzled that he offers her worship as a goddess. Convinced that she must be one of the Olympians, he rattles off a list of different goddesses' names, including Aphrodite, but the disguised deity manages to convince him that she is merely a Phrygian princess. After they have slept together in his house, Aphrodite wakes up taller, her head touching the building's roof-beam, and Ankhises does freak out, saying that he knew she had to be a goddess (Lines 172-190).
Going by a different interpretation, however, one might argue that in this scene Aphrodite is deliberately shape-shifting, in order to overawe her new lover, so that her being as tall as the house need not mean that this is her true height and appearance either, since she can (perhaps?) look like anything she wants to. In accounts of the goddess's interactions with other mortals, there doesn't seem to be any particularly noteworthy size-difference between her and them.
And Taller Still...
Ares, another lover of Aphrodite's, presents a puzzling addition to the question from a very weird thing that Homer says about him in the Iliad.
Poseidon, the King of the Sea, had consorted with his own
granddaughter Iphimedeia, and she bore him a pair of gigantic twin
sons named Otos [Otus] and Ephialtes. These kids, whose stepfather was
named Aloeus, and after whom they were called the Aloadai [Aloads],
grew by nine inches every month. By the time they were nine years old
they were each nine cubits wide and nine fathoms tall. In more modern
parlance, they were each 18 feet (roughly 6m) wide and 54 feet
(approx. 16m) tall.
... [T]he twins once
kidnapped Ares, who thus went missing for thirteen months, during
which no other deity, so it seems, knew where he was. Heriboia was the
second wife of Aloeus, and she somehow found out that her husband's
giant stepsons had Ares. She fed Hermes this bit of intelligence, and
he came in stealth-mode to rescue Ares. The Aloadai had clapped Ares
in "cruel bonds" and shoved him into a bronze urn, within which he
before Hermes came and stole him out of there. Doing what these boys
did to Ares must have required the imposition of a prodigiously
incredible amount of brute strength.
I base this on the fact that in the same poem which gives us the
twins' measurements we are told that Ares' body, when he fell down,
covered seven plethra of land, which is something like 6.3 square
kilometres (almost 4 square miles). This comes from Iliad
21.405-410, the measurements contained therein having been translated variously into English as "seven roods" (A.T. Murray, 1924), "seven
acres" (R. Lattimore, 1951) and "over an acre" (A.S. Kline's
all of which are significantly smaller than 6km2 or 4
miles2 but the least of which is still hundreds of times
bigger than the Aloadai are supposed to have been.
In comparison to Ares, they might as well have been a pair of ants! We
could perhaps explain this away by assuming that Ares, like his
cousins Otos and Ephialtes, continued to gradually expand over the
years so that he would have been significantly smaller back when they
had nabbed him some generations earlier. Or that he only grew so
extensively large on battlefields during wartime, such as in Iliad
21, which takes place in the thick of combat during the Trojan War.
~ From "Kidnapping the God of War," part of my Answer to another Question
Having said all of that, none of the rest of Ares's appearances in the Iliad seem to envision an entity who is the size of a small town, or yet bigger than the Trojan battlefield!
Indeed, the beginning of the scene in which Ares is felled by Athena wouldn't make very much sense if he were actually that size, since he is busy stripping off the armour of a dead warrior when the war-goddess encounters him. (Interestingly, though, Periphas, the unfortunate soldier being denuded of his armour by Ares, is twice referred to as pelórion, which could mean, literally, "gigantic," and is otherwise translated as "vast" and "huge".)
The only other explanation I can think of for Ares' body covering that amount of land when he falls is that we are perhaps to understand it as, rather, him having been thrown so hard by the force of Athena's blow that he crashes miles away from the original location of their fight (like in a superhero movie). Nonnus' understanding of this seems somewhat different from that. In Dionysiaca 36.14, in a clear allusion to Iliad 21, he refers to the deity as Ares Heptapélethros, "Seven-Plethron Ares," as though Homer's seven plethra are a more personal attribute of the war-god.
Furthermore, in Book 11 of Homer's Odyssey, the shade of the giant Tityos [Tityus], being punished in the Underworld, covers an area of nine plethra, and virtually every ancient author who refers to the myth (e.g. Hyginus, Virgil, Ovid and Propertius), basing his mythography on Odyssey 11, clearly understands this to mean the size of the giant himself.
In Visual Evidence
Perhaps the Same Size...
Both in the rest of the battle-scene in Iliad 21 and in early pottery paintings, Ares appears to be the same size as the other gods, at least, often depicted next to Aphrodite, such as on the Attic red-figure amphora—at the Musée du Louvre, Paris—depicting the Giants' War, in which image Ares rides in a chariot next to this goddess.
As compared to mortals in artwork, there is, similarly, nothing especially distinctive about Ares' dimensions. Here (above),1 in another piece of ceramic art at the Louvre, the god shows up in support of his son Kyknos [Cycnus], who is engaged in a duel with the hero Herakles [Hercules], who in turn is being backed up by Athena. (In this piece, it so happens, the two mortal combatants are actually a little bit bigger than their accompanying deities.)
But there are two considerations that complicate the use of ancient artwork as a determinant of the issue at hand. The one is that, for the most part, whether in vase paintings or in sculpture, all characters from the mythology tend to be depicted as roughly the same size, even, perhaps most confusingly, in scenes like the aforementioned Giants' War.
Like Ares and Aphrodite, the mortal Herakles appears in that piece too, together with the Gigantes whom he is helping the Olympians to fight against. Everyone in the picture looks like an ordinary human being, per, e.g. this detail of a sketch of the painting (see above),2 featuring the two Dioskouroi [Dioscuri] (at least one of whom at this point is still a mortal man), along with the goddess Eris, and four Gigantes.
In the friezes at Zeus' Pergamon Altar, from a couple of centuries later than that amphora, the only distinguishing feature about the physique of the Gigantes is that they are now less humanoid on account of the animal parts which they now sport, typically the huge snakes that have replaced their legs.
It might simply be that, on account of available space in the media being used, it simply wasn't practicable to depict the characters to scale, not even for the Aloadai, whose staggering specs we're provided with as early as Homer, but who appear about three centuries later on an Attic red-figure bell krater (in the Antikenmuseum Basel) as bigger neither than the goddess Artemis nor even her deer. In other vase paintings housed at the Louvre, the even more colossal Tityos is pretty much the same size as the Titaness Leto and her children Apollon [Apollo] and Artemis.
Several Orders of Magnitude Greater
Sculpture, particularly in relief, provides with us with the second concern about artwork relating to this question. We do have numerous examples of sculptures whose subjects appear in a range of sizes in the same scene, and it is clearly not a matter of depth of field.
This Athenian marble relief (above)3 is dated by Getty.edu as coming from the same century as the Attic Gigantomachy amphora at the Louvre [in the 2nd image in this Answer]. The Getty website describes it as Aphrodite with some worshippers of hers. (It looks doubtful to me that this Aphrodite could fit standing up straight in Ankhises' house.)
There are also examples with more dramatic variety among the subjects within the same scene, such as in this rural votive relief now at the Glyptothek in Munich (see above),4 showing a god and goddess being worshipped by people of different sizes. Presumably the bigger a person is the more important they are in the scene, hence the deities being the largest figures.
Raimund Wünsche elaborates on the piece on p. 118 of Glyptothek, Munich: Masterpieces of Greek and Roman Sculpture,5 saying that the god (enthroned to the extreme right of the image) is Asklepios [Asclepius] and that the goddess, standing in front of him, is his daughter Hygieia (who I notice to be significantly shorter than Asklepios, only half his height or thereabouts).
Like Herakles, Asklepios was born a mortal and only became a god after his death as a man, which only serves to complicate the matter some more, since it raises the further question—if we say that the gods are in fact gigantic—of whether apotheosis also constitutes levelling up in size.
At the end of it all, it seems quite rather inconclusive to me. The sculptures seem to display the idea that the gods are essentially whatever size they need to be such as the situation may require and that perhaps, just like DukeZhou has said in his Answer, they were not really conceived of as being a specific size.
Afterword: What Size is a Hero?
Taking it back to the written word, in Book 3 of Ovid's Metamorphoses, we hear the tale of the mortal hunter Actaeon and how he happens upon the virgin goddess Diana (i.e. Artemis) while she is taking a bath in the woods. In the scene, Diana is being attended by a bevy of nymphs who also make up her hunting party.
According to Lines 181-182, Diana "stood head and shoulders taller than" her companions. This might have been a little bit more helpful if we also had an unambiguous metric for the size of nymphs relative to humans, or if Actaeon's height happened to have been mentioned. But even if we did have that last bit of info, we also know that Actaeon, like Herakles, Kyknos and Asklepios, is himself closely related to gods. The last three are each the son of a god while Actaeon is Diana's own great-nephew, being a grandson of her twin brother Apollo.
The vast majority of characters in Greco-Roman myth, in fact, are children or close and direct descendants of the gods. Should this affect what size these semi-divine mortals are, and, connected therewith, how we might answer this Question? Ancient authors (e.g. Pausanias, Plutarch, and Quintus Smyrnaeus) might have thought so, since, according to them, many people after the Age of Heroes imagined the ancient heroes to have been humongous.
In her book The First Fossil Hunters: Paleontology in Greek and Roman Times, Adrienne Mayor points out that giant bones found in ancient times became sacred relics revered as the remains of certain Trojan War heroes like Akhilleus [Achilles] and Aias [Ajax], thus believed to have been several times bigger than the average modern human.6 So even if a goddess was the same size as one of them, she would still be literally gigantic in comparison to what we might consider normal human size today.
1. Image sourced from MythAgora. "Ares," MythAgora.com Greek Mythology, viewed 11 May 2021, https://mythagora.com/bios/ares.html
2. Abstracted from> Ravaisson, M.F.F. 1875. Monuments grecs: Une amphore peinte du Musée du Louvre représentant le combat des dieux et des géants. L'association pour l'encouragement des études grecques en France.
3. Image sourced from Getty Villa Museum. "Relief with Aphrodite and Worshippers," Getty.edu, viewed 11 May 2021, https://www.getty.edu/art/exhibitions/aphrodite/relief.html
4. Image sourced from User:Bibi Saint-Pol. 2007. Relief rural votive place Glyptothek Munich 206.jpg, Wikimedia Commons, viewed 11 May 2021, https://commons.wikimedia.org/wiki/File:Relief_rural_votive_place_Glyptothek_Munich_206.jpg
5. Wünsche, R. 2007. Glyptothek, Munich: Masterpieces of Greek and Roman Sculpture, Translated from the German by Rodney Batstone. C.H. Beck.
6. For a full catalogue of references, see "Appendix 2. Ancient Testimonia", which makes up pp. 260-282 of Mayor, A. 2000. The First Fossil Hunters: Paleontology in Greek and Roman Times. Princeton University Press, Princeton, NJ.