It's hard to estimate based on number of users and posts.
One text character uses one byte of space(perhaps a little more for misc database code), so 100MB can hold roughly 104,857,600 characters.
So if I were you I'd try to estimate average post length, average number of posts per day, post retention time etc.
But one can always start out small and go bigger later on....
Don't know if it's useful at all, but a tiny forum I run consists of a mere 10 users, has around 100 good sized posts, and is only 0.5MB in size. I guess you could scale that up relatively easily.
The thing to remember is the number of users or members will never have much bearing on the DB size, in comparison to the number of posts - it's the numbers of, and the size of the posts that will really make the difference. 100MB is quite a lot really, depending on what you consider is a busy forum!
Assuming you're talking phpBB here, the big tables are phpbb_posts_text (holds the text of the posts), and php_sessions - this can become big if you get a lot of visitors. On mine, it's bigger than the posts table, as I get a lot of visitors, but not many posters!