There is this show I used to watch called Airline or something like that on one of the cable channels. It featured Southwest Airlines and there was an episode where they made a person of size purchase a second seat. He was actually on a business trip returning back home and had to call his supervisor so they could pay for the second seat.
I have never flown SWA but will be in April. I just read their info and it appears the seats are 17 inches wide- I am not overweight at 6' 1", 190 but I am reasonably sure my shoulders are more than 17 inches wide, I guess I am sitting sideways or buying an extra seat if someone complains. I am going to promise myself never to fly anything but direct going forward....
My husband is a person of size and wears leg braces. It is very difficult for him to maneuver. SWA issued an extra boarding pass that he placed on the middle seat. The airlines were very accommodating, sensitive to his medical needs and I was grateful. (I actually had my entire seat instead of squeezed to accommodate him).
My husband is 6'5" and sometimes we can't get seats with extra leg room. We try, but you pay a price of course, and they sell out quickly. The poor girl in front of us this year couldn't put her seat back. Wish there was some solution.