Well, the thing is, the site will only store text. The compiled versions and saved files won't be that big. A huge project would still maybe only take up 70kb of space.
I figure if we use 70 kb per project and per compiled game, then that's 140kb total per project. Then lets say the average person has 5 projects, that'd be 700kb.
If we got 1000 members actively using I suspect it'd reach 7mb. 10,000 members would make it go to 70mb.
I don't have much of a worry about space. The GMG seemingly has plenty of it. Plus if we're getting too large and need to expand, I'll put up an ad to pay for it.