Ah good question, I am thinking at min a few gigs per person depending on what needs to be hosted, I would prefer not to get taken advantage of if you know what I mean.
Also, bandwidth is not an issue so long as it doesn't hog too many system resources as mentioned previously.
EDIT: I should also mention that it will be hosted on a shared server account that I own, so if someone does something stupid such as hosting porn or piracy related material, we would all get shutdown including myself.