So one of my players have achived 20thlvl as monk and increses his size to huge in combat. Currently the damage by size chart does not seem to go above 4d8, so what would be the damage of a huge size monk. Does it cap at 4d8, or goes up?
It keeps going up. Follow the damage dice by size FAQ. (Don't apply the table until after you read the text above it or you will do it wrong.)