For the past few days I've been in Paris, Bayeux, and Mont-Saint-Michel. My idea of the relative size of a medieval town, village, or village has been utterly revised. Though I've been in Florence and Rome, I considered them to be somewhat anomalous in terms of size. It was a mistake to think that way. Rome is eminently navigable on foot. Paris is the same. Bayeux is about a stone's throw across. The sizes of medieval cities hold an entirely new dimension in my mind.
Rome was an urban behemoth in its day, and even it was walkable. I never really considered this in light of making fantasy cities, towns, and villages, but really by the modern American standard they are quite small. Walkability is critical to every settlement during this period; horse-travel within cities was virtually unknown due their cost, the danger of running down a pedestrian, and the difficulty of getting from place to place in a crowded street with such a large animal. Horse and carriage travel was actually banned in Rome during the daylight hours, requiring ox drays and carts to supply the city to operate in the late night or early morning hours.
Medieval cities were much smaller and much more compact than the cities that we're used to. Every district of a city should be within reach by, at most, a fifteen or twenty minute walk. Of course, we'd have to cut out the suburbs of the modern cities that we're used to—they didn't exist, or if they did they existed as smaller ringing towns that were not contiguous. These made up the hinterlands of the city, sending products in.
Breaking and bulking was (and still is, in many senses) an important function of major urban centers. Goods travel to the largest urban center within distance where they're turned into bulk units and then shipped to another center. When that bulk arrives at another city, it is broken down and sold back to the city hinterlands. This process happens to a lesser extent at smaller regional markets, which send their bulked products to the urban center.
The medieval hinterland can't support extremely large city centers, nor can they support cities that are too close together.
Just some thoughts from Paris.