Disk space utilization will only display if your network administrator has set disk space limits on your email account.
Disk space usage is displayed under the Log Out button in both the View Mail Folders. The display shows the amount of disk space used and the maximum amount of space allowed.
When the maximum space utilization has gone over the limit set by your administrator. You will no longer be able to receive messages that will exceed your limit. Should someone try to send a message that has exceeded your limit, it will be rejected and returned to the sender stating "User mailbox exceeds allowed size: userid@domain.com".
A simple solution to save space is to empty out the "Sent" and "Deleted" folders. Under "Preferences" these folders can be turned off in User Preferences.