Set message size limit for individual user

Hi all,

I'm using Exchange 2013 CU7, the default limit message size is 10MB at organizational , send/receive connectors , users setting level. As I know the applying privilege for limit message size : organizational > send/receive connectors > users setting , right ?
How can I allow individual user to send out 20MB message without changing  organizational , send/receive connectors limit setting ?
If I have to change organizational , send/receive connectors limit setting then how to apply 10MB limit to all users (including new users in future) except some individual users ?

Thanks.