how do you know what size pack you need...
how do you know what size pack you need in terms of M/L or S/M ? i am 6"0 170 lbs
I'd put you in the M\L range.
The overall size of the pack is smaller to accommodate a smaller frame.
Being over 6' you'll want the bigger size so the hip belt fits correctly and not around your stomach.
I'm 6'2" 185lbs, the large has some extra webbing on the belt but fits the length of my back perfectly. Enjoy