IMO a smallish bbs needs at least 10MB of database storage to be safe.
If you don't plan on getting any larger than the size of my board I suppose 5MB could do if you enabled pruning though.
Also the number of members looks to have little effect on db size. Post count is the critical factor. Search functions also eat up a lot of db space (relational to number of posts as well). Advise: Before signing up with a host be sure to check out what they charge for going over your allowed db storage (and server bandwidth) limit!
Number of posts: 12177 Posts per day: 53.66
Number of topics: 903 Topics per day: 3.98
Number of users: 74 Users per day: 0.33
Board started: 14 Aug 2001 22:33 Avatar directory size: 119.19 KB
Database size: 7.72 MB Gzip compression: OFF
Go look at my numbers posted above...the members on my board post very long messages. ;) You have 5 times the amount of posts and my DB size is close to yours. Maybe there is some useless data in my DB?