Mailboxes can get enormous, considering all of the email we get on a daily basis, so here is how to check your mailbox size in the latest edition of Outlook (though this should work fine in earlier versions as well)
How to Check the Size of Your Mailbox in Outlook 2013
All your mail is actually stored in a single file for each mailbox, to see the size of the file right click on the mailbox in the left hand pane and select Data File Properties from the context menu.
You will then see a dialog appear, here you will find a Folder Size… button which we need to click.
The Folder Size dialog which should have appeared has a neat break down of all the folders in your mailbox, as well as the total size. The size is reported in kilobytes, of course, but you know how to convert those to megabytes, right? Yep, just Google it.
That’s all there is to it.