What's a Size 13 fist? And is there any chart for the sizes?

Fists dont have 'sizes' like this.

It's probably just a humorous way of saying 'a very big fist'.

(Maybe an unlucky one for the guy getting hit, too!Emotion: angry)

Agreed, it's hyperbole. It seems to be similar to a shoe size, which if you wear a size 13 shoe, you are likely to be over 6ft (2 meters) tall.