While most Gmail users have more than enough space on their accounts, some heavy email users like myself might actually near the storage space limit. Freeing up space is not so straightforward as there aren’t any real sorting options available, so my solution is to download all the emails to Outlook, then sort them by size within outlook, and go back to Gmail to delete the emails with big attachments, provided I don’t need them anymore.

http://www.labnol.org/internet/email/reclaim-lost-space-in-gmail-account/5747/