One of my cats, Sasha, discovered this tip by walking over the keyboard.
Gmail allows you a certain amount of space, currently around 10GB. You can check this by logging into Gmail via your browser, but you can also view the info in the Mail app by opening the Account Info window. I’m not sure what the menu entry is for this, but you get to it by hitting Cmd+I and selecting the Quota Limits tab in the dialog box that appears.
i suspect this will only work if you’re accessing Gmail via IMAP, which is the default setup that OS X creates when you provide your Gmail address in the Mail, Contacts and Calendars screen of System Preferences.
This probably works with any IMAP mailbox. Let me know if it works for you in the comments below.