Here is a link so that you determine exactly what you need for torque, then just select the servos.

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

FWIW - Balsa Products http://www.bphobbies.com/ has BlueBird servos. These are small, light weight (about 1/2 oz ) and go up to 57 oz of torque @4.8 volts. Regular servos weight about 1 oz each. I use them and they work fine - no problems at all. I would suggest the MG servos. I have found that the nylon gears don't hold up if they are pushed to the limit.