Bowing your head forward, find the bump sticking out on your spine at the bottom of your neck, the C7 vertebra.
Find the top of your sacrum by placing your fore fingers on your hip bones, thumbs pointing in and resting in the small of your back.
Use a tape measure or piece of string to measure the length of your spine in between, conforming to the curves of your back.
Measure a few times at different times, compare your final result with the length of a known good pack.
For an exact custom torso length you can leave a note in the message field of the order page.
A large waist belt is 81 cm tip to tip. A medium waist belt is 75 cm tip to tip.
Your waist belt, tip to tip, should be 80 to 90 percent of your waist circumference. Generally, either size will be fine for most average people, I use both sizes myself.