I'm not crazy about sitting in Coach anyway...I belong to a couple of mileage programs and instead of saving the miles for free flights I use them to upgrade to business or first class when I do fly.
The only downside to doing that is that Jim is now totally spoiled...he dearly loves to be able to recline ALL the way back, turn on the seat massage, turn on his CD player and take a 2 hour snooze. I've NEVER been able to sleep in flight (even from SFO to London - 11 hours!!!) so I just make sure I have plenty of reading material - once I forgot to bring enough and had to resort to watching the movie "Scooby-Doo" twice. (ugh...)
I usually fly United, since they're the airline we use at work and most of the time I fly it's for business. I just looked, though, and didn't see any guidelines on their website.
Southwest Air's website has a detailed page on seating policies for "customers of size".