Here is a script that will delete all the attachments from messages highlighted in Outlook 2011. To use it you can perform some sort of search in Outlook (like all messages over 10MB) and then select all the returned messages and run the script.
One issue is that Outlook will try and refresh the search results when it perceives something has changed in the database. This caused script errors when messages it was about to interact with disappeared half way through deleting the attachments of those messages (since the total message size dropped below the search filter). So the script silently ignores these errors and goes on to the next message. This can mean you have messages left after the script run that are still too large, so just run the script a few times until there are no messages left highlighted.
tell application “Microsoft Outlook” set msgs to current messages set sz to 0 repeat with msg in msgs log “Subject: ” & subject of msg set atts to attachments of msg repeat with att in atts set a to att try set asz to file size of a delete a set sz to sz + asz on error exit repeat end try end repeat end repeat log “Deleted ” & ((sz / 1024) as integer) & “KB of attachments” end tell