The rate of increase is a more interesting question than the target size anyway I'd think. If the site grows too fast, the team won't have the time to prepare hardware and staff ramp-ups in time as needed. It also prevents outsiders from integrating cleanly into the culture, because when there are too many at once, they can 'lean on each other' to retain their outsider culture, instead shifting heyurian culture away from their sheer power in numbers. This can be counteracted by good moderation, of course, but that's again where rate of increase comes in.