Hello!

Hard question to answer as every city is different and depends on its building history context, geographical and topographical environment and so on.

I would suggest you to take time and read eXeditious Retreat Press great "Magical Medieval Society: Western Europe" book. It presents a way to determine area of your city according to the population size and for each you got the "buildings density", thus you can see how your 'houses' are spread around the place. When you got the number of area and their size, it is far more easy to draw it using graphical tools (Photoshop, Illustrator and others that enables you to check for area size/surface (acre, are, square feet/meters...)). Then you would have the representation of your city.
Aside of this, I would suggest a simple but effective website (you can download the script freely to store it without being "connected") which allow you to compute (according to historical and empirical material) the size of a city, size of surroundings, size of cultures fields and even population 'classes' and business... http://www.welshpiper.com/medieval-demographics-online/

I base all my works on those tools.

Hope that helps!

Anoril