NBU DB settings of Small/Medium/Large configuration

Hello Forum,  I was wondering what best practice is for setting the "small/medium/large" confioguration for the NBU db.

Is it based on the number of clients (we have 400 windows and Unix clients), the amount of data backed up daily (10-15TB) or number of backup streams running at once (400-700 streams nightly) ?

Currently our settings are set to the "small" configuration with a very large maximum (not sure when or how that was changed):

Setting Initial Minimum Maximum

Current 200M 200M 10053M

Small 200M 200M 2048M

Medium 768M 768M 2560M

Large 2048M 2048M 4096M

We have NBU 7.6.0.3 running on RedHat 2.6 Master and Media servers

As always, thanks in advance for any info/technotes, etc