Fallout 3. In a massive interview with Gameplayer, Bethesda spokesperson Pete Hines said that, though no one had walked Fallout 3's post-apocolyptic D.C. from end to end, it's probably 50 to 80 percent as big as the world of Oblivion.
That said, it's not necessarily bad news. Fans of the series (or opponents of walking) might be relieved to hear that Bethesda is taking the quality over quantity route. Not to mention that 50 percent of Oblivion's land mass is still pretty darned big. Check out the rest of the interview for a vaultful of other non-geographic info.