You can increase the message size limit but I would strongly suggest you make sure you have the horsepower to do this or you WILL have performance issues trying to transfer large mail.
That being said, from the Exchange Powershell run the following
This command will display the current size
Get-ReceiveConnector | select identity, maxmessagesize
This will set the size to 50mb and the timout to 9 min.
Get-ReceiveConnector | Set-ReceiveConnector -MaxMessageSize 50mb -ConnectionInactivityTimeout 00:09:00
Then restart transport…
This will set all receive to 50 mb, use with caution!
