change default mailbox size, traffic and disk space for each domain
Yes, you are correct.While this may already be very obvious (so disregard this reply if that is the case), in editions that support resource management you can always create a separate subscription for each each domain and then set the limits etc... of each subscription's plan accordingly (which are then propagated to their respective assigned unlocked subscriptions).
mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa -Ne "select name from domains where parentDomainId=0 and webspace_id=0" | while read a; do /usr/local/psa/bin/subscription_settings -u $a -mbox_quota -1; done