;Create resizable Metro GUI
$GUI_MAIN = _Metro_CreateGUI("", 480, 400, -1, -1, True)
Well, I don't know if that True there means resizable...put it to false and test to maximize your GUI form.
If that doesnt work (since you dont want to play in the Metro UDF), you will have to create a small func with my code and call it just after your $GUI_MAIN = ...