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.
Growler maximum insertion depths:
Size S = 230mm
Size M = 260mm
Size L = 290mm
Size XL = 300mm