ORIGINAL: DMcQuinn

The torque measured at the servo is the same whether you use a 1 inch arm or a 4 inch arm.

The torque measured at the hinged surface depends upon the ratio of the servo arm as compared to the control horn.

So if you have a one inch servo arm and a one inch control horn (as measured from the control horn to the hinge line), then you will have the same torque on the surface as the servo.
So if you have a two inch servo arm and a two inch control horn (as measured from the control horn to the hinge line), then you will have the same torque on the surface as the servo.

But if you have a two inch arm on the servo and a one inch arm on the surface, you will only have half the torque.

Summary:

set up the servo travel so you are using all the travel you can. On the transmitter, set up the travel for 125% - 150% travel (aka End point adjust).
use a relatively short servo arm
use a relatively long control horn.

You will have maximized the torque at the surface. However, if you don't have enoug movement of the surface, then you'll have to sacriface some torque for movement.