How long is a piece of string?
What would happen on the day would depend on the prevalent circumstances, the characters involved, the available resources and the degree of common sense applied. One thing that is for sure, a quick decision would be made.
If you refuse to sit in your seat, then it would depend on the reason for refusal. If it is a reason that cannot be satisfied, then the only option may be to remove you from the flight and leave it for other parties to resolve. Obviously you need to be secure in your seat for take off, and if that could not be satisfied for whatever reason, then the aircraft would have to return to stand to enable the previous option.
If another economy seat was available, I am sure that would provide the solution. If another seat in a higher class was available, that might also provide a solution, although consideration would also be given to moving the larger customer to it.
If you get up citing a serious case of claustrophobia, then the aircraft is no longer secure and the earlier option would once again come into play.
From an operational point of view, we can utilize whatever resource we have available to us, and obviously we would seek to apply common sense to any given situation. However if a situation presents itself that causes an operational problem, we will always have the ability to mitigate the problem by removing it, for others to resolve at their leisure.