Hi Eric,
I don't know that we have guidelines for the database size. This is normally something your DBA should advise you on. You shall plan ahead for the amount of users you will be handling of course.
Our tuning recommendations can be found here:
Runtime Components Tuning - CA Identity Governance - 12.6.05 - CA Technologies Documentation
However, I don't know that we have specific info. If you need anything more specific, then my suggestion is please open a support case along with info on your database, version, amount of users, roles, etc.. - hopefully we shall be able to have further input.
Thanks,
Sagi