I’m pretty sure this is already fixed in the latest virtualmin-base 1.0-16 package.
To correct it on existing installations that are reporting this error, simply remove those two options from the /etc/postfix/main.cf file (or set them to something sane–we just don’t touch them, anymore, since filesystem size and usage can also cause Postfix to fail to start…which I think is a bug in Postfix, but there’s nothing we can do about that).