Determining Mailbox Size in Outlook

View mailbox or folders' sizes with the Properties feature

You can check the space that the mailbox and its folders occupy currently in the folder properties. 
Follow the steps below.

1. Right-click the root folder of a mailbox and click the Data File Properties.
   
   
    
2. In the Folder Properties dialog box, please click the Folder Size button.
   
   
   
   Then in the Folder Size dialog box, you can easily check the mailbox size and the sizes of each subfolder.