Arena doesn't really have a size. The cities aren't connected to each other, and the land scape just keeps generating as you walk away from cities. So the only way to get from city to city is through fast traveling.
So... it's infinitely big?
I know Daggerfall is the largest game map ever created, though. It's Beth's claim to fame

http://unrealitymag.com/wp-content/uploads/2010/05/large-video-game-worlds2.jpg