The weight of the plane has nothing to do with what size servo torque you need to safely fly it. What does have a direct requirement is the size of the surfaces/ defection angle/and the airspeed at with that will occur. I will try and post the link I have at work for figuring out what you would need for that size plane. I just ran a test for a friend of mine and we found out that he had 25% of what he really needed on rudder but 200% of what he needed on ailerons and just a little over1 00% on elevator for servos using the same size everywhere on his plane. Many people think that weight of the plane matters but it is the area of the surface that dominates the loading as well as what king of linkage your setup has.