ChimpsmithI'm 5' 10" , 40" chest and 38" waist. I use the L and from the size differences I've seen from Boreas there is very little between M & L. It's an awesome backpack BTW. I have both the 45L and 60L (again virtually no difference - just a little longer neck I guess gains that extra packing space - but the rest of the bag seems identical) and would buy another Boreas in the blink of an eye, so many nice little, innovative, touches (I had Osprey and Gregory before).