Can I restrict the size or type of files that users can transfer through the Exchange 2000 Server Instant Messaging (IM) client?

You can't restrict IM transfers directly. When two IM clients converse, they exchange messages through their respective IM home servers (possibly with the assistance of IM routers, depending on the network topology). However, when those clients transfer files, start an application-sharing or whiteboard session, or begin an audio or video chat, those communications flow directly between the two clients. This behavior explains why many corporate users can often use IM to chat but can't initiate a file transfer: A firewall is blocking the file transfer. Assuming that you don't have any interfering firewalls, the Exchange IM server and client don't have any way to restrict file transfers. However, several third-party products offer perimeter or firewall software that can block this file-transfer activity.