This depends on how many files you are backing up and how you set up ADSM to
manage them.

ADSM uses approximately 600MB of disk space for each copy of every file it
stores, so you can figure out the DB size using this number. Plus allow
about 50% overhead.

I hear that it's a good practice to limit your database size to about 20GB
per server, although this is not a theoretical limit.

Here's an example of one of one of the servers I get to manage:
Size of all storage pools combined: 13TB, DB size: 12GB, Number of nodes:
70. This server backs up a mixture of AIX and NT servers.

Hope this helps.

Dmitri