I found http://forum.teamxbox.com/showpost.php?p=5354373&postcount=7 that links to a dead ESF post, that says Oblivion is 16 sq. mi. and Morrowind is 10 sq. mi. for what it's worth.
The Morrowind = 10 square miles have been the usual answer for quite some time. A few years ago I decided to check upon it. And now I belive MW is around 5-6 square miles. And some other forum member checked up on it too on their own and got about the same as me.
An exterior cell is a square 8192 "units" per side. There are 22.1 units per foot, so a cell is roughly 370 feet per side. That's approximately 3.14 acres.
If there are 22.1 units per foot then one exterior cell is 0.0049286445 square miles. Now, there are 1404 exterior cells in Morrowind, which makes it 6.9 square miles. However, many of those cells contain just water and nothing else, so the landmass would be about 1/5 smaller which is about 5.5 square miles.
Don't know how large Oblivion is, never tried to figure it out, but I doubt it's 16 square miles too, probably more like 10.