Lets look at this logically...

Yes, the areas are not as big as any of us envisaged.
No, its not impossible to carry on as we are

If we stay at 1.5m squares the city will look more like a large town than a city.
If we scale up, the resolution on Redrobes final map will be far too low, and we will have to ask him to do this thing in quarters, if not in thirds.

Its a case of:

- do we go to 3m squares and be super nice to Redrobes in the hope that we can have a combined map of clickable thumbnails that lead to individual district maps that can be zoomed in much closer to be able to see the smaller details, or

- do we stay as we are and simply expand the city outwards as necessary?