I don't know if this will bring you comfort or not, but I'm *almost* your size (6'4" 280 with wide shoulders) and while flying is often less-than-comfortable, you will be just fine and probably won't even be the biggest guy on your flight. I've flown a ton of different airlines and sat in a wide variety of good and bad seats over the years.

Obviously an F seat is the most comfortable. Either window or aisle.
An extra-legroom economy seat on the aisle is your next-best choice because you've got the shoulder room. Just take care to let the flight attendants and drink cart pass by.
A regular-legroom economy seat on the aisle would be a bit uncomfortable but survivable as long as you avoid Spirit and the ilk.
Window seats in coach will likely be uncomfortable because of the shoulder room, regardless of the legroom.
Exit rows and bulkheads sometimes have fixed armrests, making the buttspace feel a bit narrower.

If you are seeing good value in an F seat, great. I know Flyertalkers all have slightly differing opinions on what constitutes "good value" in F. On Alaska, when I see it for my regular midcon 1200-ish mile flight for $100-125 over the coach price, I consider that "good value" and buy it. But if you don't see F seats fit whatever you'd like to spend, I'd book the cheapest Y ticket and pay for an extra-legroom Y seat ("Premium" on Alaska, Comfort Plus on Delta) and you'll be fine. It's unlikely that you would need two seats.

I always take a quick look at seatguru.com if I'm flying a type or airline I've never been on before. It'll show you rows that don't recline, rows with fixed armrests, etc. It'll also show you all of the width and pitch data for each aircraft type.