Online servo torque calculators...

http://www.csd.net/~cgadd/eflight/calcs_servo.htm

a second site with the same thing

http://www.coloradogliders.com/servo...calculator.htm

and again...

http://www.mmaa-modelairplanes.org/s...calculator.htm

and just in case all the rest go dead a 4th iteration of the same thing

http://www.flyrc.dk/servo.htm

Just be sure you provide a worst case speed of what the model will be able to do in a screaming death dive. And to prevent loading the servo you should use the number given by the calculator for servo torque required and get ones that provide 1.5 to 2 times that torque calc so you know the servo won't be operating near it's stall zone.


Enjoy!