Exchange 2003 – How do I limit Internet Email size by user

We are trying to migrate from GroupWise to Exchange and are having a problem understanding how we can limit users Internet email sizes both inbound and outbound.

Curerntly, In Groupwise we have groups created that are configured by each group to send or receive Internet email at different KB sizes. The global setting is at 40 MB but we have groups of users in 30, 20 and 10 MB size limits. I cannot determine how to set this up in Exchange unless I do it at a global level for everyone which is not acceptable.

Help!