1. Modify  Global settings

Organization Configuration->Hub Transport->Global Settings->Properties of Transport Settings
Modify   transport send/receive limits.

2. Modify Send Connector

Organization Configuration->Hub Transport->Send Connectors->Properties of Default Send
Modify send connectors default maximum message size

3. Receive Connector

Server Configuration->Hub Transport->Receive Connectors Pane->Properties of Default <hostname>
Modify receive connectors default maximum message size