If a person's size makes it so they take up two seats then they should pay for two seats. Seems fairly straightforward; pay for what you use.
How would they go about implementing this? Weighing people when they arrived? Weight doesn't always equal size. Having a row of seats so each individual would have to sit in the seat to make sure they only took up one seat? Possible. I can imagine that this would cause some passengers to be embarrassed or mortified though. Perhaps done in a private area would help.
You bet... get weighed and measured.... in private.
Just like UPS, Fedex or the Post Office does. The air lines are in the business of shipping people, so I think they have every right to charge as any other freight company.
I'm not a little guy. I'm 6'2", 205 pounds, and I'd have no problem paying more than some little person. It's going to take less energy to get that smaller person from point A to point B than it will me, and energy in the form of JP8 jet fuel costs money.