Both the WebMail interface and the MailSettings interface shows your mailbox usage.
WebMail interface:
Log in at http://webmail.3dresearch.com
Click "Options".
Under "Disk & Folder Options" select "in message listing" or "in folder list" and click "Apply".
MailSettings interface:
Log in at http://mailsettings.3dresearch.com - your mailbox usage is displayed at the top of the page as "Used Quota: [... kbytes used / ... kbytes available.(...% usage)]
Last modified 2006-03-02 00:43:52.