http://i5.photobucket.com/albums/y199/Proweler/Lore/distancemeasured.jpg

Explanation:

Map of Tamriel. I supperimposed Daggerfall on it. Doesn't match neatly so I fixed on getting the Balfira and Bretony which also gets the coast line of Daggerfall right. You can see that the Illiac has a completely different direction and that on the map of Tamriel the Illiac bay is not quite as wide.

Red & Blue line, ( 195 x 330 miles )

Based on http://www.imperial-library.info/fsg/danibenearticle1.shtml. Even though the map doesn't quite fit, it does provide a good indication of the ballpark we're in.

Green line, 250 miles.

Based on the first PGE, assuming Red Mountain starts at the bottom of the mountain graphic. Note that the scale is in the same ballpark as Daggerfall.

Black lines, 250 miles each 3.5 vertial, 5 horizontal.

So that's 875 x 1250 miles or 1.093.750 sq miles.

http://en.wikipedia.org/wiki/Europe has 3.930.000 sq miles. So Tamriel is about 1/4 of Europe.

Side note:

Using the same 250 miles and the longitude notation on the daggerfall map, Tamriel has a circumference of some 12.500 miles. About half of earth. If you were to scale the whole planet up, you'd end up with Tamriel being the size of Europe. (time 2^2 because it's sq miles.)

Another side note:

The size of Tamriel is rough estimate but I'm not going to count pixels on a map supposedly made by people without satellite mapping or grantee that the Imperial Cartography department does sufficiently accurate measurements.

Another rough estimate:

If you take the lower half of Cyrodiil and put it Hammerfell you get a box of 250 x 500 miles, Cyrodiil has 125.000 sq miles.