Can a CloudMail Mailbox Size Be Increased?

No, CloudMail mailboxes have a maximum size of 25GB and it cannot be adjusted up or down.

To check the size of a CloudMail mailbox, you'll need to log into the user's account at webmail.cloudmail.email. Once you're there, you can mouse over the bar below "Manage Folders" in the bottom left-hand corner. There is not a way to lookup CloudMail mailbox size globally at this time.