rucksack sizing chart

This is the manufacturer’s measurement advice: have fun with the measuring tape.
Length top of hip to top of shoulder
S  35-40cm
M  40-50cm

(But where does the hip end? And what is the top of a shoulder? Do they have bottoms as well?)
In general, if you don’t know about yourself that you have a very long torso or very long legs as part of your height (and people who do, do tend to know) we think:
below 161(ish!) cm: Small
161– 175cm: Medium
above 175(ish!): Large

They’re comfy, we promise …