The size of a mailbox can be obtained using the cyradm utility, by issuing the cyradmn info <imap-reference-manpages-systemcommands-cyradm-info> command.
When using quota, the size of a mailbox hierarchy under the quota root folder can be obtained using the cyradmn listquotaroot <imap-reference-manpages-systemcommands-cyradm-listquotaroot> command.
mailbox-quota