Well, there is no such thing as an "average" medieval city, particularly in the timeframe you are referencing. The 12th-15th centuries witnessed a flourishing of urban life and an incredibly significant contraction because of the Black Death.

What is more, cities in some regions were much, much larger than others and were often dependent on Geography. Consider Milan, Venice, and Genoa. The first two almost certainly existed in the first rank of cities by size. Milan was allowed to sprawl on the Lombard plane, whereas Venice was limited to the islands of the Lagoon. Genoa, another large city was bounded by mountains and was only allowed to grow in size in the 13th century when they leveled a hill. This meant that Genoa and Venice would have had to grow more vertically than Milan and Milan would have had many more "doors that open onto the street" (a common taxable status) per capita than the other two.

Check out Josiah Cox Russel's Medieval Cities and their Regions to get a nice idea of the variety we're talking about here if you're interested. I can answer more specific questions if you have them as well, but answering your original question would take a book (like Russell's). :)