How Big Was Fafnir?

Sigurd complains to Regin, when he sees the dragons footprints, that old Fafnir must be bigger than Regin has let on. But how big is that?

Well, every day Fafnir would shuffle to a nearby cliff and a stretch his neck down from it to drink the water below. The cliff was thirty fathoms (about 180 feet) high, so Fafnirs neck must have been at least 180 feet long. Dragons have disproportionately long necks, but thats still going to be a big footprint.