The easiest way to find your pole length is to flip a pole upside down, grab it just under the basket, and hold it at your side—your elbow should form a 90-degree angle. If you don't have a pole handy, measure from the ground to the top of your hand while your elbow is bent at 90 degrees. Most adults end up in the 110-125 cm range.
Your skiing style matters too. Freeskiers and park riders often go shorter (100-110 cm) because shorter poles are easier to swing around and less awkward in tight spots. Racers sometimes prefer slightly longer poles for tucking.
The biggest mistake people make is buying poles that are too long. Extra length forces your shoulders up and messes with your pole planting technique. If you're between sizes, go shorter—your shoulders will thank you after a full day on the mountain.