A 3" rudder tiller is the most that will ever be needed in any of the 50cc planes. A 5" rudder arm is waaaay too long! I use SWB rudder tillers, Tom uses AirWild. Your choice. If you are in a mood to really save money you can use the long Hitec double arm that comes with their digital servos but you can't use ball links on that one because it can bend and break. Clevises work just fine in that case. Grinding any excess length of an expensive aluminum servo arm means you can't use it later on a plane that may need a longer arm. 1/4" of aluminum servo does not weigh enough to be concerned about.

The Dubro rudder horns that are provided in the hardware package are intended to be drilled for the clevis in line with the rudder hinge line on the airplane with the excess plastic tab removed by the user. It's well covered in the text portion of the instruction manual.

If you purchase 1-1/4" aluminum servo arms from one of the manufacturers, set your linkage up at the 1-1/8" hole on the arm with the linkage on the elevator or aileron at the same relative location, as measured from the hinge center line, on the wing or horizontal stab. You can be farther out on the surface side of the linkage but don't be less unless you intend to give the flight surface a mechanical advantage over the servo. You don't want to do that.