You can apply message size limits to individual messages that move through the Microsoft Exchange Server 2010 organization. You can restrict the total size of a message or the size of the individual components of a message, such as the message header, the message attachments, and the number of recipients. You can apply limits globally for the whole Exchange 2010 organization, or specifically for a particular connector or user object.

As you plan the message size limits for your Exchange 2010 organization, consider the following questions:

• What size limits should I impose on all incoming messages?
• What size limits should I impose on all outgoing messages?
• Does my Exchange 2010 organization have a mailbox quota?
• How do the message size limits that I have chosen relate to the mailbox quota size?
• Are there users in my Exchange 2010 organization who must send or receive messages that are larger than the specified allowed size?
• Does my Exchange 2010 network topology include other messaging systems or distinctly separate business units that have different message size limits?

http://technet.microsoft.com/en-us/library/bb124345.aspx