So, what happens if I absolutely refuse to sit in my assigned seat which is partially occupied by a person of size? I stand in the aisle, ask for another economy seat, and barring that, a business seat.
This has happened to me - Pax next to me had raised the arm rest to give them more space as in now sitting in part of mine.
I quietly went to see the in-charge, pointed out I physically couldnt get into the seat. They moved me up (although I ended up with the eco meal - pah). All done sensibly whilst doors were open and without embarrasment caused.