Our bodies are all a little different, even down to the squareness of our shoulders. Check your profile in the mirror and then compare it to the shape and slant of your portage pads. You may find that a large pad has only a small area of contact on your shoulders. If it does then you can shape the pads to fit.

As for conditioning for portaging the canoe I find I need to haul the canoe daily before a trip to toughen up the shoulders. IMO preparing this way will prevent our shoulder area from becoming inflamed and tender. I worked out all winter and the years first bicycle ride on Sunday went fine. When I got back on the bike on Tuesday-OUCH my rearend. I think the tenderness was the same type produced on the shoulders when I first start carrying the canoe each year. All bikers know the first rides will make us a little tender, I think the same goes for the first days of a canoe bobbing and squashing down on the shoulder tissue.