Mailboxes form the backbone of Exchange Servers, and you need to monitor their size, traffic, and other properties constantly. To keep an eye on usage trends, you need to check the number and size of messages present in mailboxes in your organization.
To check the number and size of messages in your organization using Exchange Reporter Plus,
Navigate to Reports > Exchange Server > Mailboxes.
Under Content Reports, select Message Count and Size Summary.
In the Summarize based on field, select Mailbox.
Click Generate Now to get the results.
The message count and message size can be found under the Message Count and Message Size (GB, MB, or KB) columns respectively.