Go to your exchange client on the Java GUI, make sure you select the mailbox option at the bottom of your screen.

Then on the top right corner, right click the arrow down, then save. That will save all the mailboxes to a excel file.

There used to be a report called Exchange Mailbox Stats in store, but I can`t see it anymore. Anyways, the above will work.

Another way, same screen you showed above there is gear, click on that and export - however that will only export that is on the current screen which may or may not show all of your mailboxes. I prefer the first option I showed.