since the changelog states " * The users page no longer shows user mailbox sizes because this was extremely slow for very large mailboxes." how can you find the mailbox size for users on the server to find who is using alot of storage etc?
Tested … I can only get it to show directories, not individual mailboxes. 
What was the mechanism used by MiaB on the status page before it was removed?
Maybe try tools/mail.py user from the mailinabox directory
But, I am not a scripter so I have no clue what I am looking at. 
Though I too rather prefer having the mailbox size showing in the control panel.
Not having it show keeps the admin panel nice and fast, maybe having an environment variable in a .env file or something like
.env:
SHOW_MAILBOX_SIZE=TRUE
Or something like that.
crude method (adapt to your storage location)
du /home/user-data/mail/mailboxes -d2 -h
So just looking at this, it seems like the inboxes are stored in /home/user-data/mail/mailboxes/DOMAIN.com/USER/ in /cur/. But what about the items in the sent and archived folders? I don’t see them around that directory