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?
They don’t work. Vanguard did it way back when, with their three continent world. Each one had enough content to get from lvl 1 to lvl 50, the max, and your starting race determined your starting location. It could take up to an hour to get to friends. Even on the same continent, with a mount (before they added flying mounts), it could take a half hour of running to cross the map… and players complained so vociferously that they were forced to add fast travel options.