The technet article 'Configure Client-Specific Message Size Limits on Client Access Servers' found here: 

http://technet.microsoft.com/en-us/library/hh529949(v=exchg.150).aspx 

says the message size limit can be set on the Exchange 2013 client access server by changing the web.config file found here: 

%ExchangeInstallPath%ClientAccess\Sync\web.config 

The missing backslash doesn't bother me, but in fact, no Sync folder exists under ClientAccess on a client access server which serves only the client access server role. This folder and file does exist on the mailbox server. 

I have modified the file on the mailbox servers, per the instructions, hoping for the best, knowing much functionality has been moved from CAS to mailbox in Exchange 2013. Perhaps the technet instructions refer to Exchange 2010 and have not been updated or verified. 

Can someone please confirm with the Exchange team regarding the issue and get back to me? Please do not guess and do not suppose, please forward my inquiry to the appropriate person at Microsoft.  

Thanks

Bill C