Depends on the game. An indie title could be less than a GB, while Batman: Arkham City is pushing 20GB. Crysis 2 is 10GB. All depends on the game.
Just remember that putting a game on the SSD wont improve performance, only load times. So if its something like Skyrim (8GB) or Fallout (8GB as well) with a loading screen every time you open a door, those are the games to put on the SSD. While something like BF3 (No idea) that only loads at the beginning of levels and online play will be restricted by internet speed, that's not worth putting on there.