If you have created a send connector in Exchange 2013 then you may want to modify the size of emails you allow to leave your organization.
There are two ways of specifying how large emails should be that are accepted and sent from your Exchange 2013 Send Connector. First we will use quickest and easiest way , power-shell. By default the maximum email size in Exchange 2013 is 10mb, here we increase it to 100mb.
Simply drop to powershell and enter the below, changing the send connector name to the correct one for your organization
Set-SendConnector "YourSendConnectorNamer" -MaxMessageSize 100MB
To change it Via the GUI we need to select Mail Flow in the ECP then Edit your Send connector by selecting the pencil. As you can see image the image below you need to edit then save