Account Size Graph
If your mail account has a size limit, you'll notice a bar graph on the left hand side of the screen, under your folders, that indicates the size in kilobytes of your entire mail account (all the messages in all your folders). Above the bar, the exact size of your account is displayed, and below the bar is your account size limit. When your account exceeds 90% of the limit, you'll get a warning dialog pop-up every time the page reloads, reminding you to delete messages and decrease the size of your account. IMPORTANT: if you let your account size get too big, you run the risk of having any further email sent to you bounce back to the sender.
Related topics
Deleted messages that are not "purged" can take up space in your mailbox. For more info see Handling Deleted Messages.