Add default size quota as options for domain settings, please.
This option should be given an opportunity to establish default size quota for the domain when creating the mailbox.
This is possible since a long time - see $CONF['quota'] and $CONF['maxquota'], and the quota fields in the create/edit domain form.
If there's still something missing, please tell me more details about what is missing.
As I said, this should already work. If I'm wrong, feel free to reopen ;-)