The determination that people made about the number of users on a server was this: It doesn't matter.
However, it would probably be fair to say that no shared hosting is suitable for your purposes. Not that it would be impossible for it to work: I have no idea of exactly how much the resource consumption (mostly talking CPU and database connections here, not bandwidth or disk space) for a board that size would be (certain MODs could dramatically increase resource usage for phpBB, though, and at the 100 simultaneous users point it may well be irrelevant). DreamHost does offer promo codes and a 97-day money-back guarantee, of course, so there's no risk in trying it, except for your pains. If it doesn't work out, then you will have to look at VPS and dedicated server options.
emufarmers.com
Very little to do with either emus or farmers!