After some long and interesting grinding I have finally managed to unlock February. It fills me with a sense of pride and accomplishment to have come this far and I can only recommend the experience to everyone else. Can't wait to buy my way into March next.
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 ;)
