Disk space utilization will only display if your network administrator has set space limits on your email account.
The disk space utilization bar will display as shown below. It shows the amount of disk space used and the maximum amount available in the bar just above the New Folder, Delete, and Rename buttons.

When the bar becomes red, this means that maximum space utilization is being reached. Should someone try to send a message that will exceed your limit, it will be rejected and returned to the sender.
A simple solution to save space is to empty out the "Sent" and "Deleted" folders. Under "Preferences" these folders can be turned off if the user chooses.