Google has boosted Gmail storage space by a factor of 15 since it debuted in 2004, but power users may still find themselves bumping up against the limit (and you now have to share it with Drive and Google+ Photos too). Here are some quick, easy tips for clearing out a significant amount of room in your Gmail account.
The Gmail search box lets you look for specific file types—try searching for filename:mp3 or filename:mov to locate those larger attachments that you can really do without. The Gmail attachment file size limit is 25MB, so you may find you can free up a significant chunk of space with this method.
You can find all emails that have attachments irrespective of file type by opening the search drop-down dialog and ticking the Has attachment box. One option for Office files is to convert them to the native Google Drive formats, which don't count against your storage quota.
Plain Text and Quotes
HTML emails—with their embedded images and formatted text—don't take up much more room than plain text ones, but if you're talking about 100s of emails a day over the course of several years then it can add up. Set your next email to plain text and Gmail should remember your choice in future.
You can also save space by overriding the default setting to quote every email you respond to. On longer threads this can really start to get messy, filling your Gmail account with multiple copies of the exact same message. Erase all quoted text before replying, or select specific sections to quote.
Erase Old Emails
The Gmail search box can help you find older messages that you're never going to look at again. Try entering older_than:2y to see all emails from more than two years ago—select all, hit the trash button, and they're gone forever.
Of course you can combine this search with a label or sender to make sure these are emails you really don't need (newsletters for example). The search drop-down dialog offers some date search capabilities too.
Search by Size
Once upon a time you couldn't search for emails by size in Gmail, but now you most certainly can. Enter larger:5m to find messages of 5MB or above, or open up the search drop-down dialog and use the size option there.
In my own creaking Gmail account I found 356 emails that were 5MB or bigger, which works out at at least 1.78GB of room that can be cleared. If you regularly use your Gmail account to handle high-resolution photos or PDFs then you might come across more matching messages than you expect.
Download Local Copies
If you can't bear to lose your older or larger Gmail messages forever, download a copy to your local email client of choice. By switching on POP or IMAP through the settings page you can get your messages into a desktop program and then save or export them as required.
From the Forwarding and POP/IMAP tab of Gmail's settings it's possible to switch POP and IMAP on or off, as well as re-enable POP for all the emails you've downloaded previously. Head to the Labels tab in settings to change which labels are included in the IMAP process.