Maybe its a bug in the imap client and roundcube or the imap daemon when the mailbox size exceeds the size of normal integers. ISPConfig supports larger maildir sizes by using bigint for the databse field.
Second question, how can client check mailbox disk usage/free space via ispc3 cp? Is that possible? Maybe feauture request?
There is no such function in ispconfig. Feel free to make a feature request in the bugtracker if you need this.