Now, my big toe hits the front wall and I have to go with 13M. It has lots of storage for him and it fits his hydro flask great on the side since he uses it each day since they removed water fountains and replaced with filtered water filling stations after covid. I tried more expensive and famous options and they were tight. 5 B and that is what I ordered. I now own 7 of these! High-quality sound! Ive marked them down for comfort as theyve yet to loosen up the clamping force against my ears - which for me is a little tight. Mentioning this because it can be annoying in a rush, or would be an issue for someone not that tech savvy. I did watch the installation instruction video. I normally order extra large in clothing, but the size chart matched my measurements.