To view mailbox sizes in the Email folder list:
The table at the top of the page shows a current listing of all mailboxes with the following column information: Size, Unread Count, and Total Messages.
Total Mailbox Size is shown below the list of mailboxes, this is the combined size for all mailboxes for this user.
If there is more than one page of mailbox entries, the slider bar displays to allow paging through the entries.