A Size King is a well-endowed man who actively seeks out size queens (women who only like huge dicks). Some men who are hung become size kings because they like to be with women who will really focus on their penis and it's size. A true size king advertises his endowment to "bait" potential size queens.
"You're sure showing a lot of dick bulge tonight, Dan."
"Of course I am, Anika. I'm not going to hide it...I need to meet some size queens at the club, and I want them to know that I'm a Size King"