Try to use following script:
# for i in `mysql -uadmin -p\`cat /etc/psa/.psa.shadow\` psa -Ns -e "select concat(mail.mail_name,\"@\",domains.name) as address from mail,domains,accounts where mail.dom_id=domains.id and mail.account_id=accounts.id and domains.name='domain.com' order by address"`; do /usr/local/psa/bin/mail -u $i -mbox_quota 10M; done
Just specify highlighted domain name and quota size. Quota for all mailboxes of this domain will be applied according to specified size.