If you've never used a forum before I strongly suggest looking at what is available from your current web host... most have 1 or more available to be auto installed... my favorite of the free forums is phpbb... vbulletin is probably the best out there, but you have to buy or lease license, around $100 / yr or so...
The real issue is the performance of the webhost... a place like powweb.com might have really crappy service- it all depends on the speed / reliabilit of their database, usually mysql, that the forum is using.
In terms of the forum size, whether it's 1 or 3 or 100 subforums that's all rather easy.