a large confluence of factors will determine what space is needed. the topic of said forum would be factor #1. If your talking about a small forum with less than 250 members then 300mb is a concievable amount of space. the TFP on the other hand, will eat up storage by the GB(s) easly.
I'm sure Halx can give you an idea of what a forum on this scale will chew up. Perhaps he'll notice this thread and respond. if not, you can always PM him. he's watched this place grow, and can give a great model of growth vs. needed space i'm sure.