It's actually fairly easy to figure out how tall a horn you're going to need. But it takes some measuring of fairly short things that aren't easy to measure.

Look first at the mfg's suggested throw for the aileron. Then measure the aileron cord at the point they give for that throw. Let's say the cord is 3" and they want 1/2" up and 1/4" down. Ready with the tiny little ruler? OK......

Forget the ruler.
Almost any horn close to 1" tall will give 1/2" throw to any surface of over 2" cord easily.