Oakleaf, that's so funny because I'm going through the same thing! All my previous bikes have had 40s on them but my new WSD bike has narrower ones (I think 39s.) I don't have discomfort but I notice that my hands end up as far to the outside as they can possibly be when I'm on the hoods, and sometimes I'll find them hanging right off the side and I'm hanging on by my thumbs! I asked my mechanic about it and he measured across the back of my shoulders and said yeah, 40s would be optimal for me.

Maybe have somebody measure your shoulders (across your back) and see what the width is. It felt like he measured me clear across, from outside to outside but I'm not sure - I couldn't see!