Alchemy Arktos frames have a relatively straight seat tube, so most dropper posts can inserted completely into the seat tube (Note: small size frames are more difficult that other size frames).
As a result, determining the max travel dropper post you can use is MOSTLY affected by saddle height.
Saddle height is typically measured from center of crank to the seat post rails. This measurement is crucial because if you choose too long of a post, you won’t be able to get the dropper low enough for pedaling.
Example: Large Size Arktos 29
A: Fox Transfer 150mm Dropper Length Extended: 185.5mm (from bottom collar to rails)
B: Seat Tube Length: 483mm large size frame
Total Length: 668.5mm
This is the dropper fully inserted into the frame, so the lowest possible saddle height of a large size frame with a 2019 Fox 150mm Transfer dropper post.