As a large guy myself, do NOT choose the exit row seats! Yes, you get a few inches more leg room. On most planes you also get less width because the tray tables are held in the armrests, which don't flip up and take up more room. And to be honest, when you're large it's the width that matters, not the legroom.
I usually take a window seat myself, as I like to look out and I can also lean against the side wall or rest. However, again on smaller planes (md-80s/717s, A320s, 737s) the curvature of the wall takes up a little room, too. A lot on a plane such as a Canadair. Also, watch what your window is next to. On MD-80's /717's the last couple of rows sit right next to the engines, so you effectively don't see a darned thing. And also, some exit rows and most last rows have limited or no recline, IF that is important to you.
Personally, I have always wanted to sit way up front. WAY up front. On the left, preferably.