The video below shows how to find your total Microsoft Office 365 Exchange storage size:
You can also follow the steps below:
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
From the drop-down Email Activity menu choose Office 365 >> Groups Activity
Mouse over the rightmost point on the graph to see the actual utilized storage size
