Man has to stand for 7 hour flight due to fatty next to him.

[AOII Angel]

I can honestly say that unless a person is over the size limit for EVERY plane, it would be hard to know if you might need the extra seat. Also, airlines may have policies, but they are not publicizing or enforcing them uniformly. I fly 2-4 times a month. On the same airline I have had widely varying seat styles that significantly change the dimensions of the seat. The smaller seats are actually on older planes and tend to have fixed armrests that can't be raised.