Well I've become a HARPer, and have my own world building thing going. I'd probably enjoy helping you guys hammer out the details, just because I'm masochistic that way, but there wouldn't be any point in me wanting a land grant in it. Granted, I'd happily do the same with the Earth world project and HARP if there was enough interest, but that's a subject for a different thread, no?
It is a very pertinent question though, not least because I've gotten the impression that most world building projects suffer from insufficient scale. Among other things, I noticed this with my Earth project, I mean I've been working just on N America for something like 25 years now and in a practical sense it's still too big. I'm still discovering new things, and I still can only just barely manage to build a campaign so incredibly huge as to justify that much space to put it in.
Given that, assuming normal (eg earthlike) physics and further assuming a world large enough not to have to adjust everything to account for low gravity, short horizons, etc.... I'd think you'll want half a dozen or so people at a bare minimum. 10 or 12 might be better if you can get em, especially if you want to allow them to choose where they are on the overall story timeline as well as geographically. For example, if your world has a glacial period, you might have one or more GMs who specifically want to play in the "Ice World" period.