If you backup your CPPM instance you will get the "Total Backup Size".
If you extract your backup you should be able to see how large the databases are.
Config & Log DB backups are in the PolicyManager directory.
Insight DB backup is in the Insight directory.
I'm not 100% sure that these DB backups reflect the exact size of the DB on ClearPass though but it's something... Maybe someone from Aruba/HPE can chip in?