The only area to view mailbox capacity for CloudMail is when a user is logged in under their account. In the bottom left-hand corner, there will be a bar under "Manage Folders."
If you mouse over this blue bar, it will tell the user how full their mailbox is. The larger the blue bar is, the more full their mailbox is.
As a manager, you would need to go to webmail.cloudmail.email and log in as the user you'd like to check. There isn't a way to view CloudMail mailbox size in the Mailprotector console.