A 4 tile distance might still work. But 5 or even 6 tiles between cities migth be too far.
As someone who has proposed and implemented this idea two years ago, tested it, and uses it as of now, I can tell you it makes the game considerably better (more fun). I have tested everything from 4 to 6 tiles separation between cities; for maneuverability, 6 is the ideal, and works well in huge maps (obviously, you reduce number of civs/CS by one level), but it has two major problems. First, 6 tiles produce a "triangle" where the center has empty, unworkable tiles (workable tiles appears to be hardcoded, DLL), so from the cities production point of view, it is not optimal. In that sense, 5 tiles is better as it only produces one wasted tile in the center of the triangle.
This means that the theoretical ideal separation between cities is 5 tiles; this changes the game a lot, but I have tested the AI by watching the game on auto (Tuner), and the AI adjusts just fine to this. 5 tiles leaves a lot of open fields to maneuver, and wars become much more enjoyable; not only that, but my impression from the tests is that the AI maneuvers better with more room in between cities than what we see now in normal games. With 5 tiles, the borders between civs are really empty, and border battles become a true tactical experience. The AI performs better in part because there is no "city killing field" anymore, where two or more cities combine their bombardments to destroy an incoming army.
But there is a problem: the space between City States allocation plots is hardcoded to a minimum of 4 tiles. This means that if playing with 5 tiles between cities, some CS settlers will start the game within the "No settling possible" range of less than 5. What makes things worse is that there is no code to adapt the CS settlers to this; what you see is the offending CS settler sitting there, not settling and NOT moving, until it is captured by a barbarian (or a player). This also appears to be hardcoded, I have looked into the map generator code looking for a "magic 4" somewhere, but could not find it to change it to 5 or more.
So, bottom line, 4 tiles between cities becomes the suboptimal ideal, but even then, it's a lot better than the original 3 (plus, it reduces AI spam, which makes for a better game). I am playing an Emperor, Large, 4 tiles between cities game now, with 10/20 (standard number for large maps), and it is a great game.
So, yes, the scale was one of the biggest design mistakes, something that has been pointed out by many of us as early as 2010, so Shafer is probably only repeating what has been said many times already. I wonder if Firaxis will learn the lesson and adjust the scale for civ6, spreading resources and workability of tiles away from the city suburbs, opening the map for 1UPT to truly shine, and making for a far more strategic ownership of territory (as opposed to ownership of cities and suburbs), but that is a topic for another thread.