As far as database size, that has been discussed many times. It depends on many factors including your webserver setup, forum mods, etc. I've got one that is currently 4.8MB's in size (upgraded from v6.34) and still runs great even though it is heavily mod'ed including showing users online, etc, which are supposed to be Access forum no-no's. So in short, it all depends on your setup.
There are also some ASP scripts that will compact/repair the database as well as automate a backup on your webserver. However, I would recommend that you setup a schedule to periodically close your forum, download a copy of the database for backup, and reopen the forum. You could also use Access's compact/repair and reupload while the forum is shut down then reupload it. That is a little safer than the compact/repair script since you would have a local backup copy just in case. I also recommend that you occasionally batch delete topics while in a "forum maintenance" session which will keep the forum speed up and size down. After all, you will have a backup copy of the database should you ever need those old topics. I keep several "archived" copies zipped up on my local pc which dramatically decreases the file size of an Access database.
One easy to implement and fairly cost effective solution is to use Second Copy 2000 (www.centered.com) to automate the process of backups in conjunction with Web Drive (www.southrivertech.com). This makes backups a snap. Of course you could use a "manual" way of setting up web drives using TCP/IP bindings but there are some security issues to be aware of.
Remember, you are only as good as your last backup...there's my ramblings for the day. Hope they help somewhat.
Edited by xeerex