15" Ftw
I have personally used a 17" MBP in coach on various airlines (Southwest, United, etc) and it just doesn't work. While the footprint may be only slightly larger, remember that it makes the screen that much larger as well, which means the base will have to slide closer to you to achieve the proper viewing angle since the screen will hit the back of the seat in front of you. This makes for a very cramped user experience.
And if the person in front of you decides to lean back and take a nap, forget about it. The only way it will work if it is on your lap, not on the tray table.