users
We have about 900 exchange users globally. In our location (Mexico) we have an Exchange 2007 server with only 22 users. In this server, we have 8 users with 10 - 15Gb and remaining users with 2 - 6GB.
15GB users don't give us any problem, my point is Exchange can easily manage and deal with big mailboxes. If you have 5000 users, you must keep mailboxes small (1-2 GB max) and setup limits, restrictions of all colors and types.
Users