I am a big guy, 6'2" and over 300lbs. I have flown all over the world both domestic and international flights, I have probably covered most carriers and am yet to find one that I didnt fit. I have been on a few that were uncomfortable and I would certainly not want to sit in the middle seat, but generally speaking you should be fine.
If necessary you can get an extension belt, ask quietly as you get on rather than risk being embarrassed when you find out you need one. If you have the opportunity to pay for extra legroom, do so (You can always tell your colleagues that you had points or received a free upgrade for some reason). Make sure you dont take too much hand luggage, ideally, just the one bag that you can put in the overhead. You dont want to be sharing your limited room with luggage. My only other advice would be to refuse the meal, I can fit in the seat, but there is no way that the little tray is going to work.......I figure, 1 missed plane meal is not going to hurt