The Elder Scrolls IV: Oblivion -- 16 square miles
The Elder Scrolls III: Morrowind -- 6 square miles
The Elder Scrolls II: Daggerfall -- 62 square miles
-saw that somewhere, not exact again. And Fallout 3 is supposedly about 80-90% the size of Oblivion, and New Vegas I have no idea.
But yeah, Daggerfall isn't infinite.. it just might as well be.
Daggerfall is "http://www.gamesas.com/eng/games/games_daggerfall.html", that is about 200,000 (not a typo:
two hundred thousand) square miles. Also, about half of Oblivion's 16 square miles is behind an invisible fence (outside off the borders of Cyrodiil), as opposed to Morrowind. To be fair, the 6 square miles figure of Morrowind include the underwater parts just around the island ...
So it's basically: Oblivion's usable space is about 50% more than Morrowind's, and less than 0.01% of Daggerfall's size.
And I'd like something in-between. About 1% of Daggerfall's massive size would be just fine. Somewhere around the size of Trinidad, Guadalcanol or Bali. Or Delaware, for that matter.