One thing to remember is the way servo torque is measured. The advertised torque is measured 1.00" from of the center of rotation. The longer the arm, the lower the available torque. Short servo arms produce more torque.
Keep them as short as practical.

The instruction manual or plans of a well engineered model will call out the servo type, the arm length the surface deflection on all control surfaces. Also, the neutral position of the servo arms will be shown, in order to establish the proper aileron differential.