This article explains how to check the size of your email account and the size of each subfolder in Outlook 2010, Outlook 2013, and Outlook 2016.
- From the File tab, under Info, select Cleanup Tools, then Mailbox Cleanup.
- Click the View Mailbox Size button. Select the Server Data tab of the Folder Size window.
- 1,000,000 KB = 1 GB. Use google to automatically convert KB to GB
Be sure to look at the Server tab, not the Local tab. The size of your folders listed on Local do not count towards your email storage quota, since these files are being stored on your 'local' hard drive.