The dropper post length is dependent on a couple of factors; the first being your saddle height and the second being the maximum seatpost insertion depth which you’ll find listed below. Using the values below and the charts supplied by the dropper post-manufacturer you will be able to figure out which dropper post length is best for you.
2018-2020 Instinct carbon:
Size S = 230mm
Size M = 245mm
Size L = 280mm
Size XL = 280mm
2018-2020 Instinct Alloy:
Size S = 200mm
Size M = 230mm
Size L = 280mm
Size XL = 280mm