There are several places to check when configuring message limits in Exchange 2003.

• Exchange Manager > Global Settings > Message Delivery > Default
• The Default SMTP Virtual Server > Message
• Also in Active Directory > Users: Properties > Exchange General > Delivery Restrictions
• There is also an "Allowed Sizes" option on the Content Restrictions tab if you use an SMTP connector (under Routing Groups)
• You can also configure limits at the user level: Active Directory > Users: Properties > Exchange General > Delivery Restrictions

More Info

• http://www.petri.co.il/setting_size_limits_for_messages.htm
• http://www.msexchange.org/tutorials/Set-Size-Limits-Messages.html