There are already some huge maps out there, Just Cause 2 and 3 both have maps at around 1000km2, and those games are beloved by their players. But if the next Cyberpunk game was announced with Night City now being the size of an actual large metropolis, say like New York, would you say that’s too big? What determines what “too big” is?
An Open World is only too big if it requires loading screens at transition points that aren’t natural. An Open World can have an insufficient density of relevant content, where exploring it has too little marginal utility to the player, and therefore it is ultimately not useful to exist.