A number of our mailboxes are going to be migrated to another system,
so we need to determine the actual space usage for the Users' mail.

I tried running a Mailbox Statistics report but I'm having trouble making
sense of the Size amounts. [The report shows both a Size (bytes) and
Size (KB), which don't seem to match. Also, if I check, within my
Client - Tools / Check Mailbox Size, I come up with an entirely different

How can I determine actual usage for select mailboxes?