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.
Altitude Powerplay carbon:
Size S = 166mm
Size M = 205mm
Size L = 230mm
Size XL = 268mm
Altitude Powerplay alloy:
Size S = 193mm
Size M = 231mm
Size L = 270mm
Size XL = 308mm