I was sat in the back of the plane which isn't a big deal. Boarding was almost complete when the last passenger came aboard. The only seat open was in the middle seat to my left. This person was on the big side and after they sat down the armrest couldn't go down and this person took up 30% of my seat. Because of the flight being full I had no choice but to sit sideways for 2.5 hours as this person shared my seat. There should be a policy if you can't fit in a standard seat, you should have to pay for two. If there are no extra seats available then that flight isn't for then. I have seen people asked to leave the plan for petty reasons or because they were pick because the flight attendant. This has happen on many flights for me. I feel if I paid hundreds of dollars for my seat, I should be able to use 100% of my seat. I know some people might find this insensitive, but you wouldn't want to pay to go watch a movie at a theater and have to share your seat. I think there should be changes made to make sure everyone is comfortable in the seat that they paid for.