Calculate how Much Space Your Outlook Mail is Using

Open outlook go to Tools Mailbox Cleanup. 

 The Mailbox Cleanup dialog box open ,click  View Mailbox Size. You can also use this same dialog to easily find old email, or items that are larger than a certain size, so you can delete them and reclaim some space.

In the Folder Size dialog will show the amount of space being taken up on your local machine by each folder.