The video below shows 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 OneDrive Files is displayed and click on the View more button
Click on Site usage
Mouse over the rightmost point on the Storage graph to see the actual utilized storage size
Click to learn more about Onedrive Backup