Technically, it's going to be something like Elite, or X. In planetary terms, I wouldn't be surprised if it is Daggerfall. Wiki says that game area is around 63,000 square miles - Vvardenfell has 9 sq. mi. and Oblivion is about 16 sq. mi. by comparison, IIRC.
I'm unable, it seems, to dig anything up re: game area sizes.