These instructions apply to Outlook for Microsoft 365, Outlook 2021, Outlook 2019, Outlook 2016, and Outlook 2013.

View this Microsoft article What's the size of my mailbox? to find out more about checking the size of your Outlook mailbox.

Note: Be sure to look at the Server Inbox, not the On My Computer Inbox. The size of your folders listed on "On My Computer" do not count towards your email storage quota of 100 GB, since these files are being stored on your local hard drive. 

All Exchange Online limitations may be viewed in this Microsoft article Exchange Online Limits.