I was looking for a way to delete large attachments in my Gmail. Gmail doesn't offer a sort by size feature, so I figured I'd do a little Googling. It led me to this:

Sort Gmail by Subject, Labels, and Senders to Organize your Inbox

In doing that, I found this nice trick (from the page above):

Though Gmail provide hundreds of thousands of inbox memory (at present 7400MB), then to you can run out of space. In such a case search for “has:attachment from:me label:sent” which will list mails sent by you and have a attachment. You can safely delete them to get some extra space.