First, start Outlook
Click on “Edit”, then “Find”, and then “Outlook Items”
Checking for Messages with Large Attachments, and Deleting them Properly In Mac Outlook
Make sure that you select “All Mail” and then click the drop down arrow on Attachments
Then click on “Greater than 10 MB”
Change the search criteria from 10,000 to 45,000. Delete any mail that is found by pressing Shift + Delete, and clicking “delete”
Check the “Deleted Items” folder to make sure that the mail was actually deleted, if everything worked you should not see the message you item(s) you just deleted