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.
2017-2020 Element Carbon:
Size S = 177mm
Size M = 215mm
Size L = 253mm
Size XL = 291mm
Size XXL = 329mm
2018-2020 Element Alloy:
Size S = 155mm
Size M = 190mm
Size L = 220mm
Size XL = 250mm
Size XXL = 329mm