You don't need a larger horn unless you want more throw on the the rudder. Make sure your cables are attached in the same position on each side of the servo and at the rudder or you will get uneven throw. The same rules for travel apply to cables and push rods.

The tension on each of the cables is the same when centered. No extensions are necessary because the distance between the servo horn and the rudder horns remains constant throughout the travel, the only thing that changes with input is the location of the of the horns.