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?
Basically, how much of the world is interesting/fun.
For example, Fallout 3 doesn’t do a great job of this, as much of the world is baren with no story or gameplay. Half of the world feels like it could be cut out without much loss. The Yakuza games on the other hand, have smaller worlds but they feel massive and fun because there’s always something to do moments away.
The work-around is to make travel fun, so the “empty-space” is just more gameplay. The Just Cause games are the perfect example of this. All the movement mechanics are quick and satisfying, from the grapple and parachute, to the driving, to the OP wingsuit.
I would argue that Fallout 3’s map is ridiculously tiny.
On the other hand, the world of Fallout 4 feels very cramped; you can’t go 5 meters without encouraging something. Bethesda’s games are interesting in this aspect – the worlds of different games are built similarly, but they differ in some small parameters (as in the density of Fallout 4), so they’re ripe for comparison.
Personally, I feel there were two peaks in Bethesda’s worlds – Morrowind and Skyrim. Both for different reasons.
As a Morrowind enjoyer, I find Skyrim to be too shallow. There’s 7 weapons in the game, 7 spells, and nothing really to do.