In chapter 10 of the Saga of the Völsungs, Regin is trying to convince Sigurd that Fafnir is an average size dragon:
"Young am I," says Sigurd, "yet know I the fashion of this worm, and how that none durst go against him, so huge and evil is he."
Regin said, "Nay it is not so, the fashion and the growth of him is even as of other lingworms, and an over great tale men make of it; and even so would thy forefathers have deemed; but thou, though thou be of the kin of the Volsungs, shalt scarce have the heart and mind of those, who are told of as the first in all deeds of fame."
Source: The Story of the Volsungs, translated by William Morris and Eirikr Magnusson
When the pair ride to slay the dragon, however, Sigurd realizes from Fafnir's tracks that the dragon is a tad larger than Regin lead him to believe. Regardless, Sigurd - with a little help from the ever-present Odin - slays Fafnir.
I'm trying to get a basic understanding of the mechanics of dragon slaying in Norse mythology, and the comments on Fafnir's size piqued my interest. How large did the Norse imagine dragons to be? Do any of their legends give explicit information on the size of dragons?