The standard install is 1.6 GB. That contains the game, most common maps and the tutorial missions.
If you play online, the Matchmaker map pool is another 540 MB. You will probably end up trying out some more of the hundreds of available maps anyway, so I'd recommend to allocate at least 4GB.
Keep in mind that this is a full 3D RTS game and those graphics don't come out of thin air ;)