I'm 6'5" and always thought I wore a large pack. Makes, sense right? Wrong! I measured myself according to the directions on the Gregory website and turns out I wear a medium backpack. I'm short-waisted so it makes sense.
I've gone through many backpacks and this one is a dream. I can carry 45 pounds like it's not even there. The articulated hip belt and dialed-in shoulder straps are the best. I carry very little if any, weight on my shoulders. The hip belt also snuggles into your lumbar area for added support. The lid has a ton of storage space and the water bottle pocket on the side of the pack hangs away from the pack so condensation doesn't get anything wet, and the pocket tucks away when not in use.
Also, the pack doesn't stick up too far past your shoulders. It's shorter and wider than typical packs and I love being able to tilt my head back. It still holds a ton of gear.