Household size has decreased considerably over time. We use a theoretical model for investigating the relationship between household size and the size and internal structure of a city. In a stylized model, single-person households occupy in equilibrium the central parts of the city, while two-person households live in the less central parts. Differences in incomes and costs of transportation drive the results. Using a numerical example we find that a city with an exogenous population shrinks in geographical extension if the share of single-person households increases. The theoretical results are confirmed by data from two selected cities.