I want to limit the size of messages that a user can send internally. I set the Message Transfer Agent (MTA) size to 1024KB on each of my Exchange servers, but email messages larger than the size limits still go through. Why doesn't Exchange restrict messages larger than my set limits?

The most likely reason you're seeing this behavior is that the MTA doesn't touch every message. In an Exchange Server 5.5 organization, the MTA transfers messages only between servers, so if one user sends an 11MB message to another user on the same server, it doesn't go through the MTA. Also, unless you turn on the RerouteViaStore registry subkey, the MTA doesn't necessarily see messages that pass through the Internet Mail Service (IMS). Turning on the subkey forces the message through the MTA, letting the MTA apply its limits. In addition, you must make sure that you have the same MTA size limits on all MTAs in the organization; if you don't, Exchange might route some messages through an MTA that doesn't have limits in place.