To search for email which has large attachments, in Outlook:

1. In your list of folders, scroll down and right click on “Search Folders“
2. Click on “New Search Folder“
3. 
4. Then select “Large Mail”
5. Click the “Choose” button and change the number to “10,000 KB“
6. 
7. And click “OK“
8. Outlook will now search all email and display a list that has attachments totaling more than 10mb per message.