Mike Garton's soaring column in Model Aviation a couple of months ago had lots of useful information on the subject. Analog servo accuracy suffers a lot under heavy loads but digital servos do much better under heavy loads. for torque requirements go to
http://www.multiplexrc.com/calcservo.htm

You may also want to consider nonaerodynamic loads, such as a dragging rudder in a ground loop or a dragging flap on landing.