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 carbon:
Size XS = 200mm
Size S = 225mm
Size M = 225mm
Size L = 245mm
Size XL = 245mm
Altitude alloy:
Size XS = 170mm
Size S = 200mm
Size M = 250mm
Size L = 250mm
Size XL = 250mm