1. Open the MessageStats Reports website.
2. Click on File and select custom report.
3. On the Data source page select "Inventory Reports -Mailboxes"
4. On the Fields page add the Organization and Mailbox Size (KB) fields.
5. On the Group Page add the Organization field to the Grouped fields.
6. On the Format page click the Advanced Summary Calculations button. (It doesn't really look like a button but it is in the middle of the page.)
7. Place a check in the Mailbox Size (KB) / Average check box, Uncheck Show detail Records.
8. Click Finish to display the report if you want to save the settings click File | Save As and give the new report a name.