1. Click File, select Info 
  2. Navigate to Tools > click Mailbox cleanup…
  3. Check the Find items larger than 250 kilobytes radio button, then click Find.

  4. Click on Size to sort the search list by the size column, this will assist in locating the emails with the largest attachments.

  5. Double click the email with the large attachment to open it > right click on the attachment > click Remove Attachment.