Minecraft! *hides knees*
A note on minecraft though, 8x the size of the world is the maximum area a map can cover. It doesn't generate the entire surface of a planet when you start a game, but only generates new areas when the player explores them (so if you back up your game, explore eastward, revert to the backup, and explore eastward again, the area will be different)
On a more serious note though, ArmA.2's default map is about 225km^2. None of which is separated by load screens or generated on the fly, meaning a single scenario can cover the entire map without any interruptions. I assume this is what you're asking.
Of course, that's just if you're restricting this to stuff where it's detailed sufficiently for the player to be able to walk around, otherwise, it'll probably be some space sim. A few hundred kilometre's tend to look kinda meagre next to an entire star system, never mind an entire galaxy... so I think "the grand prize" for the largest playable environment would probably go to Spore, where you have an entire galaxy to dick around in.