My temporary rudder extensions normally only add chord to the rudder, maintaining the original height. I find that subfins and subrudders are particularly effective on bipes, since they are in clean air while the airplane is flying at a high angle of attack, unlike the rest of the vertical tail. In my book, getting really solid controllability extending into stall territory requires a vertical tail area of about 15% of the wing area, for a normal tail moment/wingspan proportion, and that the larger the vertical tail area, the better the controllability.