One possible strategy (the easiest if you do not have a lot of "special users"
you will use Mailbox storage policies. policies will apply to all your mailbox-enabled users. (cfr http://support.microsoft.com/?kbid=822938
For specific users, from active Directory users and computers MMC, you can change the send/receive limit. simply select the user account, right-click and select properties. Go to Exchange general tab and press Delivery Restriction button. The per-user setting will override the mbx storage policy.
Note : Exchange attributes are visibile within Active directory if the Exchange Admin tools are installed ..