If it was that big, and detailed to the normal levels of TES games, then I'd say your game would be released in 2013 by current standards.
I disagree. It can be made MORE detailed than Oblivion, in less time.
Detail and content is more important than size IMO
I disagree.
Big important cities that look like little villages, dungeons and ruins spaced a few dozen meters from each other, mountains that are not real sized, etc... THESE THINGS detract more from immersion then less detail and content.
Anyway, you CAN have a much bigger gameworld with lots of detail and content.
Generate most stuff procedurally than TWEAK things you want to be unique.
Just like the real world is all procedural, and only a few things are really "unique".