There are formulas for it, and ther is even an on-line calculator for it. (I'm sure someone will remember the URL...)

The online calculator takes into account Area, aspect ratio of the control surface, (it asks for "span" and "chord" of the surface to get the area) expected maximum deflection angle and expected maximum airspeed. If I remember crrectly... it does not take aerodynamic balancing into account (which would reduce the servo power needed.)
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http://www.csd.net/~cgadd/eflight/calcs_servo.htm (found the calculator)