If you're running out of space in your Gmail or Google Apps inbox, the quickest fix is nixing the largest attachments you've got stashed. You can't do that from Gmail's web interface — unless you've enlisted Find Big Mail's help.
Find Big Mail creates three IMAP labels on your Gmail account — My Big Mail, My Really Big Mail, My Ultra Big Mail, corresponding to minimum file sizes of 500KB, 2MB and more than 2MB. Using those labels, you can further refine what you're looking for in Gmail's search:
label:my-ultra-big-mail MP3 would help you clear out old songs your friends used to send you before web file transfers were so easy. Otherwise, you're free to run through the labels, select multiple emails and pare down your account storage.
You can use an actual size sorting tool if you hook up Gmail to Outlook, Thunderbird or other IMAP email clients, and there are Gmail-as-file-storage tools that get it done too — though those tools can sometimes lead to temporary account holds from "unusual use". Find Big Mail authorises through a temporary OAuth token, sorts your mail in a few easily understood labels, and only stores the sizes of your messages on its own servers, according to the team. It's a free service to use.