there's a trick we use. Lift the arm up and it will give you a little extra room. If possible get an end seat, you'll have a little extra room with the aisle that way.
Obviously, your hip size will be the true determination on how well you fit into the seat.
Depending on the FA, that little trick may not work. If you are in another person's space, and you can't put the arm down, the airline CAN make you purchase another seat. They don't always, but they can.
i am a size 3x -26 i ususally fly jet blue and have not had a problem yet, although i always get sooo nervous that i won't fit. i do have to suck it in a little to get buckled at first. i didn't even know they had extenders, how do you go about getting that?