Just to be clear, the game ES IV: Oblivion takes place in the Tamriel province of Cyrodiil and reportedly in the realm of Oblivion (hell). There is no info on how big Oblivion is, but one statement by Todd Howard says that Cyrodiil is 40 sq mi.
Only 16 sq mi of the Cyrodiil province will be in ES IV according to the statements made.
In comparison, the island of Vvardenfell where the main game of ES:III Morrowind takes place has been said to be 10 sq mi. That statement was made before the Bloodmoon expansion was released, and I haven't seen anything which talks about how much the area of Solstheim would add to the 10 sq mi figure.