The answer to your question is fairly simple, but difficult to explain.

If you are using massive deflections on a .90 or larger size model, you will need 200+ oz/in of torque per surface. If you are flying that same .90 airplane at low speeds with no more than 20 degree deflections, you could get by with 40-70 oz/in. Rudder requires a bit more because it is a larger surface.

An example: I just built a .90 size plane, (ARF), which called for a 1.0 size motor (go figure). The mfg recommended JR 9411 digitals on everything but rudder, and 8411 on rudder. (I think 8411's are 90 oz/in and 9411's are 70...not sure). Because I intend to use this plane for 3D, I installed a 1.25 engine, 120 oz/in servos all around and a 300 oz/in servo on rudder....all regulated at 6V. So it REALLY depends on what you are going to do with it. This same airframe would fly just fine on standard servos if I only flew it flat and slow. (I hope I am making sense with this example...or do I just sound like I want to over-power everything....?).

Anyway....when you blow-back your first servo, you will learn very quickly where the line is.

Good Luck!!