The maximum size of an email that can be sent or received can be set either a Global, Connector, Server or indeed the individual user's level. For most purposes it is best to set the limits at the Global level to avoid confusion. The default global limit is 20Mb.
Get/Set Global message limit
Look for the MaxReceiveSize and the MaxSendSize.
To set the global send and receive limits to 15Mb:
Set-TransportConfig –maxsendsize "15Mb" Set-TransportConfig –maxreceivesize "15Mb"