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?
There is no open world that is too big. They can only be too small.
However, the quality of an open world is not predicated on the size of the open world, but rather what is actually in it.
And this doesn’t mean that open worlds must be drowning in content, as the quality of the content itself also matters, and certain worlds that are large and empty can still be interesting due to its traversal being good, or the sandbox nature of a large empty world.
Some of the worst examples of open worlds are the kind that are just filled with isolated little fetch quests; busywork that’s all marked on the map with no element of organic exploration. Or the kinds of open worlds where nothing actually happens “organically” without the player starting it.
The best kinds of open worlds are the ones that emphasise exploration and/or have background systems governing the world in some way (i.e. factions that interact with each other without the explicit involvement of the player).
I feel Daggerfall would be too big without the quick travel systems, but thats the only game Ive felt dread about slow travelling to distant locations