Hi,
You can use a du command. Emails are stored at /var/qmail/mailnames/yourdomain.tld/emailadresses/
per domain storage, use this: du -hs /var/qmail/mailnames/yourdomain.tld
per email storage in domain, use this: du -hs /var/qmail/mailnames/yourdomain.tld/*
BR,
Andras Jokai
www.799.hu