I am 6'1" tall, weigh over 250lbs. I am not going to couch any of this in cute terms. I am not "pooh sized". I'm overweight. I won't even go into my feelings on using such a term to cover up admitting that one is overweight. Let's call it what it really is. Overweight. Fat. Obese. Calling it by any other term is nothing more than denial.
I am obese and I have had no problems fitting in seats on Air Tran, Southwest, British Airways, Alaska Airlines, Air Canada, Air Canada Jazz, or West Jet. These planes are fitted with standard seats installed at both the Airbus and Boeing factories -- nothing special. I have recently flown on the CRJ 700 series on Air Canada Jazz. This is one of the less roomy planes in the Air Canada Jazz fleet. No problem. My friend is bigger than I -- he is approaching morbid obesity -- he had a seat belt extender and had no problem either on any of the Canadair Regional Jets.
Find out what equipment the particular airline flies to the destination on the route you are considering. Go to
www.seatguru.com. The website is invaluable for learning about seating configurations, seating pitch and seat width.
Unless one is morbidly obese, I would suggest that one has little to worry about in flying on airlines. If one is morbidly obese, then fitting in an airline seat should be the least of one's worries.