There’s a few places to change this and that’s global settings as well as the connectors.
To adjust your transport send/receive limits.
Global Settings
Organization Configuration->Hub Transport->Global Settings->Properties of Transport Settings
Then you must modify the connectors found here:
Send Connector
Organization Configuration->Hub Transport->Send Connectors->Properties of Default Send
Here you can adjust your send connectors default maximum message size
Receive Connector
Server Configuration->Hub Transport->Receive Connectors Pane->Properties of Default