Having flown an international flight next to a guy who was obese and spilled into my seat for the entire 9 hour flight... I think I can speak authoritatively on the subject...
Anyone over a certain size should have to pay extra and then not put anyone in the seat next to them. That way fatty gets the room he needs, no one else is subjected to intrusion into their space and the airline gets its money.
I should have gotten a partial refund since I didn't get to use the entire seat that I paid for. I would have moved but the flight was full so I was stuck...literally. That fat fuck had me pinned in my seat. He had to get up so I could get up. It was the most misearable flight I've ever had.
Nothing personal against fat people, but if they're going to impose on my space then either they need to pay for it or I should get recompense for putting up with their fat ass.