How is the biggest upset determined??

a lower rated person that wins of a much higher rated person.

the game with the biggest difference in rating in a tournament is the biggest upset. 

So I wonder what the biggest upset in a full length FIDE-rated game has been, based on rating difference?

Harrelson 1/2- 1/2 Kasparov?

Everyone can win in chess, the rating doesn't matter; however, the biggest upset from mine is that you are in a winning position (Either a mate in one, material advantage, etc.) and then you blunder and lose. That's a tough upset.

A draw is not a upset, but a good result from Harrelson :)

Lol. Woody's game doesn't count. It wasn't a serious game. And he has Yasser Seirawan helping him.

I mean a serious tournament game between FIDE rated players.