Very small. Most homes were 1 or 2 room shacks. A medieval family was somewhat larger than most families today. I would say an average of 4-6 kids, however there was a high infant mortality rate. In addition, extended families would often share a house. I would guess by some drawings that I have seen that 20'x30' would be a good starting place for a lower income house. Then move up from there. Many homes would be 2 floors, and often were rentals, with one family on the first floor, and another on the second.

Probably half of the city would be these smaller, lower income homes. They are the "grunt" work. From there you would have merchants and shop owners whose home is also their business, so they would be bigger. Then you steadily move up till you get to some of the larger townhouses which will be owned by wealthy merchants, nobility, and others who can afford them.