The biggest Boskone was 4200, and it can't really be said to have worked.

I think professionally run events of any kind (and StarCon/Starfest certainly qualifies) can avoid the 5000 limit for 3-day cons, which is imposed by the impossibility of getting anyone to volunteer at all.

AnimeBoston has moved to Memorial Day weekend for '06. But you're right, the '05 con, once they'd moved to the Hynes where the fire marshal would let them pass 4100, had something like 7500 people.