So based on this if Fearabbit is correct in saying:
[...]
The world map can be calculated to being over 53 000 square km or over 33 000 square miles.
This would make the map slightly larger than LotR Online and about half the size of Daggerfall.
Whoa, let's not get overexcited, and check our math, shall we? If one cell is 1172 m2, that's 0.1172 km2 = 0.014 km2. This times 3900 equals 53 km2. You're wrong by a factor of 1000.
53 km2, however, is pretty huge, so let's recheck our facts! The quote actually only says that Morrowind's cells are 81922 units big. We know, however, that Oblivion's cells are 40962 units big, that's 4 times less than Morrowind's cells.
So if we divide our 53 km2 by 4, we get around 13 km2, which is probably a good estimation. (With both Morrowind and Cyrodiil being slightly larger than that. The numbers in the quote in one of the posts above are rubbish, however, because that guy simply took a rectangle of cells which has LOTS of area outside the actual game's boundaries. Oblivion should have about 16km2 and Morrowind somewhere between Skyrim and Oblivion.)