Frequent flyer here!  Airplane seats are uncomfortable, they have mediocre to bad legroom, the cushioning tends to be rock solid, most (though not all) barely recline, having said all that... I can comfortably fit in between the arm rests without spilling over at all.  I actually have to be careful because my coworker always tries to put me on a middle seat between him (Jabba the Hutt) and another coworker (The Notorious White B.I.G.). 17-19 inches is fine, my car seat is no wider than a plane seat... I don't spill over when I drive, I also don't spill over when I fly... they're just too big to be considered normal humans.