A lot depends on the rudder size, hinging (gaps) and weather or not it is pushrod or pull - pull. A properly engineered surface (rudder, elevator, flap, aeil etc.) is equally important.

Smaller oftern requires less and larger normally requires more. Pull - pull is the most efficent, but is rearly used in todays models.

By the way, for those of you new to the hobby, it wasn't that many years ago that a 100 oz servo was a mosnter.

Some of todays airframe manufacturers do not go through the exhaustive design as testing that should be done prioro to the release of a a new model. Too often maxi servo torque is used to overcome that lack of proper design. In the end problems are often incurred.

I had a BVM Bandit which is a very fast airplane that used pull - pull on the rudder. I employed a Futaba 3002 55oz metal gear servo and worked flawlessely for 400 flights. This is a very well engineered system the didn not require a "maxi" servo. The elevators on the Bandit were equally well engineered. I ran 70oz servos on them and again, never had any problem. I guess it's the old saying; how well is it engineered?!?!

Actually the torque of the servo isn't as important as the gear train. As I was taught many years ago, servo specs usually never reveal anythig about what's inside the servo.

Hope this helps some.