Find your C7 vertebra by bowing your head forward and finding the big bump sticking out on your spine at the bottom of your neck or around shoulder height.
Find the top of your sacrum by placing your fore fingers on your hip bones (the Posterior Superior Iliac Spine), 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 inbetween, conforming to the curves of your back.
Measure in inches if you like.
A properly sized pack is infinetly more comfortable but if you are unable to measure 50cm is aproximately large, 45cm medium, and 40cm small.
It is useful to know your waist circumference at hip belt level.