A full game you can get like CoD4, Assasins Creed, NFS Pro Street, etc. can easily exceed 2 GB of HD space. So full fledge games like those are pretty big.

The biggest size of a game that I've seen a 3DGS game be is about 800mb for it's full game release. Then again small projects can be as small as 50mb, or even 5mb depending on its contents.

I didn't help much leaving you with a broad range of numbers, but if you knew exactly what you were putting in the game and how big you planned to make it then a better estimate could be made.