The steps below show how to find your total Microsoft Office 365 Exchange storage size:
1. Log in to your Office 365 environment as an administrator, and click Admin.
2. Then click on Show all...
3. Click Reports > Usage.
4. Scroll down until the Email Activity is displayed, and click View more.
5. Click the 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