Change Outlook 2011 max attachment size
Increase the “maxRequestLength” to 51200000 in web.config (found in ‘ClientAccess\exchweb\ews’ & ‘ClientAccess\owa’).
Increase the “maxReceivedMessageSize” 51200000 to in web.config (found in ‘ClientAccess\exchweb\ews’).
Open command prompt & run the following:
cd \Windows\System32\inetsrv appcmd set config "Default Web Site/ews" -section:requestFiltering -requestLimits.maxAllowedContentLength:51200000 appcmd set config "Default Web Site/owa" -section:requestFiltering -requestLimits.maxAllowedContentLength:51200000 iisreset
Requirements Exchange 2010 SP3 (Will not work with eg. SP1)