Changing from retention baased to quota based

Hello All,
Management wants to convert from a 90 day retention policy to a 10GB/user quote based retention policy.

Eventually I'm trying to figure out the how many GB a post office can handle before experiencing performance issue. Somewhere I read that users should be moved to another POA when the POA size reaches 150GB...

Has anyone seen some sort of calculator for this scenerio?

If you are running a quota based system, how are you configured?
Recommendations?
Partition type used for storage?

thanks
Brad