Generally speaking standard (45oz) servo's will fly a typical sport plane up to the 60 size class. If you are doing some serious airobatics then a larger servo for the elevator and rudder would be called for. For just general knock around flying standards would do. Otherwise Bubba has given you some real good ideas. The Hitec HS-475 is a decent servo to look at. It can be bought mail order for less than 20 bucks and has 61 oz of torque at 4.8 volts. I feel they are a real bargain. And if you feel the need for a little insurance these will give it to you. Good luck.

Mark Shuman