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?
I don’t think it can be too large, but like others have said, there has to be enough quality content in each location you can visit to compensate for the vastness of the open world.
It be amazing if you could go inside every single building/dungeon/etc. and have every one of them chockablock full of things to experience, like they did with Elder Scrolls 6, but look how long it took for that game to come out…