The company I work for has 15 employees and perhaps 25 legitimate mail boxes between users and service accounts. I have at least 5 users that have mail boxes of 500 mb or larger (with the winner being a 3.5 gig box!).
My boss is not concerned with this problem--he read somewhere that Exchange 5.5 has "no limit" to its possible size. I need to have some compelling Microsoft reason to mandate a "cut back" of size and objects in the Exchange sever in order to avoid possible hazards.
1. How about backup? How long does it take you to run a backup of that information store, and does it all fit on one tape? How long would it take you to restore that system if the unthinkable happens?
2. Exchange 5.5 Standard edition does have a maximum size limit for the information store--16 Gigabytes. The Enterprise edition is only limited to disk space available, and you don't specify which you're running.
3. Searching: It takes a lot longer to search for individual items in a big mailbox.
4. Legal liability: Have they seen what happened to Bill Gates when an old e-mail message was brought back to him in court? Some of your users may be saving old messages that they shouldn't be.
Other than that, I can't think of any particularly compelling reasons to limit the size. If people need that many items, then they do. However, I suspect you just have an office full of packrats who are reluctant to ever delete anything or empty their deleted items folders (have you checked to make sure they know HOW to empty their deleted items folders?).