Perhaps you should check out seatguru.com. I'm told it lists the seat info for most airlines. You have to fit between the arm rests, with them down, and not taking up any of the seat next to you. I think most economy seats around around 18" wide.
ETA: I once flew next to a very large lady who immediately upon sitting down yanked up my arm rest and proceeded to take up (literally) half of my seat. She looked at me and said (I was thinner back then) "Sorry skinny but you're gonna have to make room!" I was terribly uncomfortable and embarrassed to make a fuss, but there was no way I could fly 4.5 hours that way. I ended up summoning a flight attendant and asked to be moved, but instead the woman was told she had to buy an additional seat. It became very embarrassing for me and for her.
So my point is, if you find you are going to need an extra seat, I personally would find it MUCH less traumatic to get it before you go so that the sort of unpleasantness the lady and I experienced doesn't happen to you.
"Giving up because of one setback is like slashing your other 3 tires because you got a flat."
"If you don't sacrifice for what you want, what you want becomes the sacrifice"