The power a windvane self steering system can deliver is directly related to the size of the pendulum blade, and the height above the water. The higher the mounting, the longer the pendulum rudder, the higher the force the system can generate.
But the the actual force depends on how light the boats steering actually is. An un-balanced barn door rudder will require more force to steer the boat than a fully balanced spade rudder. The lightness of the transmission in the rudder system is also a factor.