Everyone knows that the goddess Frejya's chariot was pulled by cats, but what kind of cats are we talking about? Are they giant sized house-cats? Were there any puma-sized cats in ancient Scandinavia?
There are actually two references to her cats in Gylfaginning (part of the Prose Edda):
Sessrumir, her hall, is large and beautiful. And when she travels, she drives two cats and sits in a chariot. (Faulkes: 24)
…Freyr drove in a chariot with a boar called Gullinbursti or Slidrugtanni. But Heimdall rode a horse called Gulltopp, and Freyia her cats. (Faulkes: 50)
The passage about Baldr's funeral in Skaldskaparmal (also part of the Prose Edda) uses the word fress for Freyja's cats, which means tom-cat, although apparently it could also mean "bear" which led to some confusion. (Modern scholars translate fress as "tom-cat" in this case.) Another reference to Freyja's cats in Skald., saying that Freyja can be called “possessor of tom-cats", uses the same word.
The two references to Freyja's cats in Gylfaginning use the word köttr, which could also mean a marten or weasel. This makes a little more sense, as you can see how cats, weasels and martens could be lumped together. However, both parts of the Prose Edda were written by the same person, Snorri Sturluson, so presumably he meant the same thing in both cases.
Cat-lovers are partial to the idea that Freyja's cats were the ancestors of modern Norwegian forest cats, which are large, powerful cats (like Maine Coon Cats). The Scandinavians did have cats as pets, but whether Freyja's cats were simply medium-sized furry animals or actual cats is open to question, although the detail about the volva's outfit in andejons' answer is suggestive.
Quotes from Edda, Snorri Sturluson/Anthony Faulkes, Everyman Press, Penguin, 1992.
They were tomcats.
When she goes forth, she drives her cats and sits in a chariot.
(Note that the original actually specifies "two cats"). This is repeated in XLIX, when the gods travel to Balder's funeral:
... Freyja drove her cats.
This is everything I've found in the primary sources that explicitly deals with Freya's cats.
The largest cats that is and were found in Scandinavia is the Eurasian Lynx, but it is unlikely that there is some confusion here. Considering that Frey was pulled by a boar, and Thor by goats, we should probably not speculate too much along the lines that they can not have been ordinary cats.
However, some scholars have wanted to link this with the Mediterranean goddess Cybele, who had a chariot pulled by lions (but this is likely coincidence). There is also a passage in the Saga of Eric the Red, in which a völva appears and does sejdr. She has her clothing described in detail, including a hood of lambskin, lined with white catskin, and gloves made of catskin, which had white fur turned inwards. Since Freya was associated with sejdr, this has suggested a connection with her cats. So, if you want to depict the cats, making them white would not be a totally wild guess.
The last passage on scholarly speculation is mostly out of Gro Steinsland's Fornnordisk religion.
Read something somewhere of a story of Thor running across a magic male cat who was singing to two grey kittens. When Thor asked if the cat was the father it said yes and explains that he methrew a girl cat and they had the kittens and the mother cat left him as a single parent. The cat then asks Thor for help. Thor accepts thinking to give the kittens to Freya. The magic cato then turns into a bird and flies away.. Speculation is then they are the "Russian Blue" breed. Being as they had a magical father they could possibly be strong enough to pull a chariot, maybe change their size at will?