Say you used a 7955 Hitec servo at 333 ounce-inches. If your servo arm were 3, you'd have 333/1.5 = 222 ounces of cable tension at max rated servo torque. 222/16= 13.9 pounds. So I'd say 20 pound would work. I might go with 30-40 pound for the added safety factor if it's not much heavier. I think I remember 40 pounds being the cable most people use up to 40% aerobats.

PS: freeair, the largest servos are only about 9.5 kg, not 12. And nobody uses a 1" rudder servo arm or horn so the servo torque and cable tension are not 1:1. You need to divide your servo kg by 1.5-2 inches depending on the rudder servo arm/horn length.