This is only available for Outlook clients using MAPI
- Right click on your Exchange Mailbox, which is the very top level, and choose 'Properties for Mailbox ' from the pop-up menu.
- On the Mailbox Properties window click the 'Folder Size' button.
- On the Folder Size window you will be able to view your total mailbox size in kilobytes (KB). You divide the total number by 1024 to figure out how many MB [megabytes] are being used. This window will also show you the size of each folder individually. If you don't need to know the exact size you can just remove the last 3 digits (e.g. 9000KB = 9MB)