the only thing i don't like about the airlines in these cases is that if you buy multiple seats because you want comfort, they get pissed because they want to fit more people on the plane... i'm a big dude, 6' and 270, and can fit into a single seat easily - however, on long flights i'd totally buy two seats if they'd let me for some extra leg room... but they won't.
why not just make sure you get in an emergency exit row or first row of seating, sometimes its free, sometimes it cost like $15-30 but me being 6'3+ try to do it whenever i can