Before the 1800s, the sizes of cities were limited by:

The range of people could walk on foot is the right answer.

Explanation:

Urbanization is the method by which large groups of people become enduringly concentrated in moderately small stretches, growing cities. Before 1800, the sizes of cities were limited by the areas that people could move on their foot. However, after the 1800's, cities began to grow so quickly in the United States. There were several reasons for this, one such as migration. As soon as the crowd extended the areas of the cities began to expand.

The distance people could walk on foot.