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?
Depends how full it is, how interesting is it (note this is not the same as full), how fast you can travel, and how fun movement is.
There’s a lot of elements to open world and a lot of devs get the balance very wrong. You end up playing in a map rather than the world.