when you create a virtualdomain, is it possible to limit a postmaster to 5 accounts, or 10 accounts, depending on what you set them? also, mail quotas.

Yes. Try using VQadmin (http://www.inter7.com/vqadmin/).

you can find all command line options in inter7.com site under vpopmail


you can also read the man pages that come with vpopmail install

Please read the INSTALL, README file when you are installing qmailadmin