Dropper post length is dependent on a few 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.
Instinct Carbon 27.5" wheels:
Size XS = 185mm
Size S = 210mm
Instinct Carbon 29" wheels:
Size S = 210mm
Size M = 235mm
Size L = 250mm
Size XL = 280mm
Instinct Alloy 27.5" wheels:
Size XS = 225mm
Size S = 225mm
Instinct Alloy 29" wheels:
Size S = 225mm
Size M = 250mm
Size L = 275mm
Size XL = 300mm