You can search by size in gmail in the web site. Also, you can connect any pop/imap email client and sort by whatever you want. Even if you don't want to use

You can search by size in gmail in the web site. Also, you can connect any pop/imap email client and sort by whatever you want. Even if you don't want to use Thunderbird full time, spend 10 minutes once a year to get it connected and do some mailbox cleaning and you can pretty much solve this problem. Alternatively, you can just archive off X years off mail and store it off of gmail. That way if you ever need it you can always dig it up.