How to generate Dashboard Summary Report?
Perform the following steps to generate a Dashboard Summary Report:
Select Exchange Online Reports from the available vertical tabs. Then from left pane application, select Dashboard Summary.
For demonstration purpose, "Top Users by Mailbox Size" report has been chosen.
On selecting "Top Users by Mailbox Size" report, a window is displayed as shown below:

Columns to be displayed on the Grid can be selected from the dropdown control. By default, all columns are displayed.
After selecting the desired column from the dropdown control, press Refresh button to display.
Follow the above mentioned steps to run the other Dashboard Summary reports.