Biggest, as in most content, stuff to do, etc. I don't think any other single player game comes close to Skyrim. "Fuel" supposedly has a 5000 square miles world, but due to the lack of variety in the environment, and stuff to do, it doesn't feel that big, at all. A large map needs a large amount of content, and, of course, a purpose (Placement of a city, or the only place where you can find some mobs, or some NPC's, or items.). Without those things, it's just an annoyance. Fuel, which has the biggest, non-randomly generated map of any single player game (I think only one or 2 MMO's have it beat; Guild Wars isn't 15,000 square miles, like the image that's floating around suggests, nor is Lord of the Rings Online 30,000 square miles -- Lotr:O, the original maps, are probably smaller than Azeroth in World of Warcraft. I think Arena and Daggerfall have "randomly" generated maps, more like a Minecraft world seed I suppose.), doesn't really feel bigger than, for example, Test Drive Unlimited.
A game world needs content and interesting stuff to make you appreciate the distance, why you went from one point to another, to make exploration fun. Basically, a game world should scale with the amount of content. I used to just want large worlds, but I've since realized that it doesn't work very well.