I am planning to buy the marathon for my ds, we are going to wdw in sept and I want to use it on the plane, a friend at work told me that just because the seat is FAA aproved that does not mean that it fits on the plane. We will be flying continential, thanks for any tips!

I don't know if that's because it's so big or if all carseats do that. (We've found it helps to take his shoes off, BTW. He doesn't kick as much and it's not as hard.) 
If I had not almost been it tears I would have been laughing too.