There are a number of places to limit message size in Exchange 2007:

1. Organizational limits: Apply to all Exchange servers in the Organization
EMC > Organization Configuration > Hub Transport > Global Settings tab > Transport Settings > Properties > General.

2. Receive Connector limit: Receive Connectors are only used to receive messages
EMC > Server Configuration > Hub Transport > Receive Connectors > Connector > Properties > General.

3. Send Connector limit: Send Connectors are used for sending outgoing messages to the internet or particular address spaces (domains).
EMC > Organization Configuration > Hub Transport > Send Connectors > Connector > Properties > General

4. Mailbox limit: Individual recipients (mailboxes, etc) can have their own limits to bypass the Organizational limits (only for internal messages).
Recipients > Mailbox > select mailbox > properties > Mail Flow Settings > Message Size Restrictions

5. Global Settings: Global settings can impact Exchange Server 2007 recipients (if upgraded from Exchange 2003)
Exchange Server 2003 Global Settings > Message Delivery > Properties
If all Exchange 2003 servers were removed Using ADSIEdit.
In Exchange Server 2007 SP1 changing Organisation Transport limits automatically change Global limits.

6. Active Directory SiteLink limit: In Exchange Server 2007 SP1, you can also set maximum message size limit on AD Site Links.
Exchange Server 2007 uses the AD Site topology to determine the least cost paths. If the message size to be delivered to a remote AD Site exceeds the limit on the AD Site Link, message delivery will fail.
By default, the MaxMessageSize on AD Site Links is set to unlimited. This can be changed using Exchange Management Shell (EMS):
Set-ADSiteLink “<Site Link Name>” -MaxMessageSize 20Mb

7. Routing Group Connector Limit: Routing Group Connectors are used in co-existence scenarios to transfer messages between Exchange Server 2003/2000 Routing Groups and the Exchange Server 2007 Routing Group. Messages exchanged between these Routing Groups should be below the message size limits of their respective RGCs. The default is set to unlimited. To set the MaxMessageSize on a Routing Group Connector:
Set-RoutingGroupConnector “<Connector name>” -MaxMessageSize 20Mb

Source: http://exchangepedia.com/2007/09/exchange-server-2007-setting-message-size-limits.html

Exchange Server 2007