This website has helpful info such as seat size on different aircraft. Some planes have seats that are an inch or so bigger and that might make all the difference.
http://www.seatguru.com/
Since you seem to have a lot of anxiety about this, maybe you should try to buy two seats, if you can afford it. The extra expense is simply an investment you make in yourself, similar to buying larger size clothing or supersizing a McD meal - these higher expenses related to your larger size are acceptable so why not an extra seat. I'm not trying to be cruel, just trying to put things in perspective. If you can't afford two seats, well there are a lot of people that can't afford even one seat.