for a really small game I would think of something in the range of 6mb but big games can become easily 20,100,200(?) mb. It mainly depends on how much and how big models and textures you gonna use.
edit: as example myst3 had a gigant disk space of 8gb!!
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.
actually that did help, i guess, it helps t o know the small games because the limit on size for me is 150mb, whihc should be pelenty, as long as models dont get too complex
