How big is Halo 3?

This makes no sense. Anyway, if you mean how much can you do. There's 5 things to do. First, there's campaign(story mode) where you fight the covenant. Then, there's matchmaking where you have competitive games against people around the world through the internet that are set up by the Bungie (the makers of the game). Then, there's custom games this is like matchmaking but you set up the game and invite people to play. There is forge where you pick maps from the game, build stuff in them, and save them. Then, you can play on your maps in custom games. Last, there is theater where you can watch videos of your previous games. You can take pictures and record clips in them.