Here is what you need to know.

What size engine?
What Airplane are you flying. Wingspan?
What type of flying are you doing?


Generally, a 40 size sport or trainer plane needs only a standard Ball Bearing servo. 50oz of torque or so.

a 90 size plane you might go up to 75-100oz of torque. If its an aerobatic plane you might use a metal geared servo on the rudder.

on a 1.20-140 size plane you might go with 125oz. IF you are doing significant aerobatics like 3D you might consider metal or carbonite gears.

In general, if you are flying an aerobatic plane performing agressive aerrobatics you might go one size too big just to be sure you have enough torque.

I am generalizing quite a bit. You can make it more complicated but its not rocket science.