The statement "Most American families are smaller than..." is incomplete, so I'll provide some context and information about family sizes in the United States:

In the United States, the average family size has been decreasing over time. Here are some key points about American family sizes:

1. The typical American household (not necessarily a family) currently consists of 2.5 people.

2. The average American nuclear family (parents + children) is around 3-4 people.

3. In recent years, single-person households have become more common, accounting for about 28% of all households.

4. The median number of children per family in the US is approximately 2.

5. About 29% of households consist of married couples with no children.

6. Around 30% of households are made up of individuals living alone.

7. Non-traditional family structures, such as single-parent households or same-sex couples with children, have become more prevalent but still represent a minority of total households.

Given this information, a more complete statement might be: "Most American families are smaller than the average family size in the 1950s and 1960s when the typical family had more children."

If you'd like to complete the original statement with a specific number, it would likely be something like "Most American families are smaller than 4 people" or "Most American families are smaller than 3 children," depending on exactly what aspect of family size you're comparing.