I had a customer that had problems sending emails over 10mb via Entourage EWS via an SBS2008 Server.
So here is how to fix it.
In addition to running the appropriate cmdlets to change message size limits in Exchange, you'll need to edit web.config found in C:\Program Files\Microsoft\Exchange Server\ClientAccess\exchweb\ews. Open with Notepad (or your favorite text editor) and search for maxrequestlength. Change the value to reflect the size in kilobytes desired, then save the change.
For good measure, edit web.config located in C:\Program Files\Microsoft\Exchange Server\ClientAccess\owa.
Once both files have been edited open a command prompt and run the following commands:
appcmd set config "SBS Web Applications/ews" -section:requestFiltering -requestLimits.maxAllowedContentLength:51200
appcmd set config "SBS Web Applications/owa" -section:requestFiltering -requestLimits.maxAllowedContentLength:51200
51200 KB = 50mb