That's where things can get sticky. Doing it scientifically based on the "average" BMI or other statistical form of measuring what amount of space will accommodate a body seems the logical way. I know that B6's economy I will sit comfortably in any window or aisle seat but avoid middle seats as I am very broad and don't want to invade my seatmate's area if avoidable. I remember flying Cross Air to visit my brother in Italy and the middle section I believe wad 4 across and I was stuck between two very large guys and not being a midget myself I had to sit with one arm in front and the other behind me in a twisted position. the FA took pity on me and precut my meal so I could eat it with a single hand. Lufthansa Y seats on their A320 family & 747-8i are torture to me but for some reason their ERJ-190s I'm fine for legroom and AS's 737 in regular economy over 3 hours would be cramped particularly when ti comes to seat pitch/legroom.
Essentially, to a point the regulation would evaluate what exists in the market and make the average +/- a bit and go from there. All existing craft would be grandfathered but ideally it would keep things from going too far.