That depends on a couple things..
1. what is your budget
2. are you going 6 volt or high voltage.
If budget is no problem, then look at the brushless servos
My general rule of thumb is 100 ish oz-in for elevator, and 200 ish oz-in for aileron and rudder.
http://www.futaba-rc.com/
pull down the servo tab and you can look at the spec's and prices.