How to Clear Up Space in Your Gmail Inbox
I was up to 87% full in Gmail, but there was no way I was going to search for every e-mail that had an attachment (type “has:attachment” in the search box), because I couldn’t sort by size. So I could spend hours deleting e-mails and gain only 1% back. I did search for “filename:.mov” to get rid of one kind of file that I knew was big. Even that would take forever, so I cast about for another way.
I finally found it: use Outlook to download headers only, then sort by message file size, then delete messages in Outlook, then sync up again. Would you look at me now!
Full instructions here.