That number can't be correct, Access can't go over 2gb. Perhaps you meant mybaseforum.mdb 2845500 Bytes.
The only way to shrink an Access .mdb file is to run a compact process on it, I think you can do that from the admin pages of the forum. Otherwise, use Access itself.
Fairly busy forums usually stay around or under 30mb. Lightly used forums will probably be around 5mb or so.