I think cities should NEVER be limited to the generic 21-square pattern, and be allowed to expand as they please. The catch, however, is that cities must maintain a level of culture that is strong enough to bind a city together, or some of the outlying citizens may break away and form a city of their own, emigrate, and so forth.
In addition, cities and their citizens should physically exist on the map, and so another disadvantage of oversized sizes in the earlier eras is that resource production may not reach the city in a timely fashion, since the level of "connectivity" a citizen has with his city depends upon the number of movement points required to reach the city. A citizen with low connectivity would be inefficient in that it may take 2 or more turns for his resources to reach the city where it is actually exported or consumed.
This generalized model, however, allows for large metropolis cities such as New York and London to develop with advances in technology. Therefore, large cities will develop in a more reasonable and realistic manner.
In any case, my entire model is largely explained in my
Urban Sprawl Model . I will also soon be integrating it into my general economic model, the
UET II .