Have you tried to set the default position to maximized? (Right click a channel/query window on the channel bar while it is maximized and choose Position > Set As Default).
Beyond that, you can try something along the lines of:
on *:active:*: { window -x $active }
What that will do is it will maximize whatever window you click on. So, join them minimized, then click them and this will make sure they open maximized. You shouldn't need to do this, though. The default window setting should be plenty.
