This cropped up a few years ago when I used to work for a well-known national carrier, I think it must have been after one of those reality shows was screened.
We looked it up and the official rules stated that such a person should buy two seats. My supervisor said that he had never had to do this in the 20 years that he had worked there. The unofficial rule was to find two adjacent seats (upgrading the original pax if need be to create the space.) If none could be found the large pax would be politely informed that it would be more comfortable to travel later and they would be rebooked f.o.c. It would all be done with no drama.
Having to ask people to move from their prebooked seats was a daily occurrence, generally they were ok with it as long as the replacements were compatible. I suppose it is really down to how the PSA deals with it, being officious and offhand was a definite no-no.
However, things may well have changed for the worse now.