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 there’s a too big for a simulation type game world, go all the way. But for more directed game styles that are narrative driven or more carnival ride than simulation don’t make it boring use techniques from past games; the keeping distant landmarks in view outside like in New Vegas, or hilly landscapes to obscure stuff to discover like in Zelda or Skyrim. Bad examples would be like traveling between towns in daggerfall or those monuments in the middle of nowhere in starfield.