Set message size limits in Exchange 2007 and 2010
Organisational Limits:
EMC > Organization Configuration > Hub Transport > Global Settings tab > Transport Settings > properties > General tab.
Receive Connector Limit
EMC > Server Configuration > Hub Transport > select a Hub Transport server > Receive Connectors
Select a connector > Properties > General tab
Send Connector Limit
EMC > Organization Configuration > Hub Transport > Send Connectors
Select connector > Properties > General tab
Link: http://exchangepedia.com/2007/09/exchange-server-2007-setting-message-size-limits.html