I believe setting the quota at the mailstore database level is best. That way you can plan exactly how many mailboxes you can put in each store so as not to exceed a specific mail store size. And when someone’s needs change you can move that mailbox into the appropriate store. I think it would be way too great an administrative headache otherwise. As far as sizes, you have to decide what is best for your organization. For us we have directors and CEO’s at 1 or 2GB depending on need. The rest of the organization is 500MB. That is extremely generous compared to many other organizations I have seen.