Just have a look on the airline's website and you'll find info about aircraft type (also tells you about food), although the vast majority of US domestic flights are now on narrow bodied aircraft, so as eloquently put by Rambuster it's just different degrees of bad.

I haven't looked, but i'd go as far as saying that all flights from BOS to SEA will be on narrow bodied aircraft with standard F class seats of 40" pitch and a 2x2 config.