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.

The Altitude requires a Minimum Insertion Depth of 100mm of seatpost. 

Altitude Carbon 27.5" wheels:

Size S = 202mm

Altitude Carbon 29" wheels:

Size M = 250mm

Size L = 280mm

Size XL = 310mm

Altitude Alloy 27.5" wheels:

Size S = 192mm

Altitude Alloy 29" wheels:

Size M = 240mm

Size L = 270mm

Size XL = 300mm