How do I find the current size of an Exchange mailbox using Outlook? View The Full Article 1. From Outlook, rightclick on the name of the mailbox then leftclick on Properties. . . 2. Click on the Folder Size . . . button. 3. You will now see the Folder Size window that shows all the current data. 4. The two most common folders that fill up the quota allowed are Deleted Items and Sent Items. If you are receiving email warnings saying that your mailbox is over quota, check these two folders and try deleting any old messages or messages with large attachments to clean out your mailbox. Sizes are given in kilobytes KB. Divide each number by 1000 or move the decimal places to the left to get the size in megabytes MB.