Hi techtiger,
Thank you for your question.
In addition above suggestion, we could use Exchange server Mailbox Statistics to monitor mailbox database. The tool retrieves mailbox statistics which includes Folder Count, Total Items, Associated Items, Deleted Items, Total Items Size, Deleted Items Size,
Oldest Item Date, Newest Item Date, Items Age, Mailbox Age and Quota details. It also includes an additional option named "Archive Statistics Planner" which will let you search mailboxes and provide statistics report with specific dates.
We could refer to the following link to download it :
https://gallery.technet.microsoft.com/office/Exchange-Server-Mailbox-7dd53529
If there are any questions regarding this issue, please be free to let me know.
Best Regard,
Jim