Good advice ijgordon! I once could not sit in my assigned seat because my large seat mate was taking up her seat and most of mine as well. I was told to de-board and wait and as I stepped off the plane, the flight attendant told me that if they could not find me another seat, I would have to take the next flight. It was a lesson learned that even though my overweight seat mate should not have been my problem, once you are off the plane, it essentially does become your problem.