The steps below show how to find your total Microsoft Office 365 Exchange storage size:
Log in to your Office 365 environment as an administrator, and select Admin
Then click on Show all...
Choose Reports >> Usage
Scroll down till the Email Activity is displayed and click on the View more button
Select - Mailbox Usage tab.
Mouse over the rightmost point on the graph to see the actual utilized storage size
Click to learn more about Exchange Online Backup