This doesn't sound too hard to script.
Using Java or LotusScript, whichever you're
most comfortable with, in a server agent, you could scan your Domino directory
for server names and filepaths of user mail files, locate the databases
in question and find out their sizes. Compile a report of the results in
a rich text field of an email message and have the agent mail it to you.
It might be someone has already done
something like this. Did you check openntf.org?