Just ran into the "413 Request Entity Too Large" issue myself and wanted to post the fix in my case. Of course, it was a single radio button:
Ensure that the Autodiscover virtual directory is set to "Ignore" Client Certificates. The deployment I was working on was somehow set to "Accept" and it caused all kinds of craziness, including completely taking down autoconfiguration of Outlook, and registering that error when running Test-OutlookWebServices |fl.