Wait...there are other sizes mentioned in the game and in other places, such as:
looking at her face over a distance of thousands of miles
That is in one of the books that deals with the Four Score War in Oblivion, and is about Vivec looking at Almalexia from Vvardenfell to Mournhold. This would mean that Morrowind itself is at least three thousand miles from the House Dres plantations on the border to the Sheogorad region.
He and his tribe live here in stately and exotic palaces built from massive timbers of Valenwood oak, whose territorial borders are only a few hundred miles away
That is from the Imperial Library, and seems to hint at a similar sort of scale to the source mentioned above. Torval seems to be right on the border with Valenwood on some maps, though, which means that it is at least two thousand miles across.