Originally Posted by PTravel

There's a simple answer to the OP's question:

You have a right to the seat that you paid for. If you wind up next to a passenger of size such as described in the OP's post, refuse to accept your seat. Stand in the aisle and the FA you're not moving until either you're provided another seat or the passenger of size is removed. If you're told to deboard and take it up with the gate agent, politely refuse and say, "Please have the gate agent come on board to address this situation." If you're told that there are no more seats and you'll have to take the next flight, tell the FA or the GA, "Oh, so this is an IDB (involuntarily denied boarding)?" You do not have to give up half your seat to anyone.