How much hosting space do you truly need for your forum? I've never known how much space to purchase, nor do I know how to calculate this. Let's say, for example, a forum of decent size (~100,000 posts). How big would it require?
I believe WebGurubb - a forum with 26,000 posts- uses around 2Gb in total. The database it a hell of a lot smaller than that though (perhaps a few hundred mbs). That may give you some idea. 🙂
Start with maybe 1gb, 2gb or even 5gb. I never understood people who want to get over 50 or 100 gb of space. Unless you're corporate, you'll never cone close to those numbers. Resellers or vpses not included, speaking strictly for s single site.