I have 2 "rules" for DB size.

1.  Restore time - if it will take longer than your recovery window on your
TSM server to restore the database, your DB is too big.  A good estimate of
restore time is 50% longer than the backup - 2 hours to backup the
database, 3 hours to restore.  If your maximum outage on your TSM server is
6 hours, and the database backup takes 4 hours, you're cutting it very
close.
2.  Backup media - I don't like spanning my TSM DB backup across 2 pieces
of media.  If you have something small (3570, 3995, 8mm, etc) and it takes
more than 1 volume to backup the database, it's too big.  If you do have to
restore the database, having to find two (or more) pieces of media to do it
can make things more complex that I'd want to deal with.  This is a
personal opinion, based on personal preferences, but it can be a good lever
toward getting better/bigger/more advanced media technology, too.

Nick Cassimatis
nickpc AT us.ibm DOT com

Today is the tomorrow of yesterday.