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 27.5" wheels:

Size S = 210mm

Size M = 235mm

Altitude carbon 29" wheels:

Size M = 235mm

Size L = 250mm

Size XL = 280mm

Altitude alloy 27.5" wheels:

Size XS = 225mm

Size S = 225mm

Size M = 250mm

Altitude alloy 29" wheels:

Size M = 250mm

Size L = 275mm

Size XL = 275mm