The Census Bureau report New Single-Family Homes Sold Not as Large as They Used to Be by Philip Thompson (12/21/2020) notes:
The average square footage of new homes sold in the United States increased from 2,457 in 2010 to 2,724 in 2015 but dropped in 2019 to 2,518, according to the U.S. Census Bureau’s Characteristics of New Housing.
Note that the title is bit misleading based on the first sentence of the article. Homes are smaller than in 2015 but still larger than in 2010. Interestingly (also see graph)
Despite the decline in average square footage, the share of homes with four bedrooms or more that were sold increased from 41% in 2010 to 49% in 2019.
Now, note the switch to comparing to 2010 as the number of 4+ bedroom homes is down from 2015. Plenty to explore here for stats/QL class, for instance what is the relationship between home size and the number of bedrooms?
The link in the first quote brings you to a page with numerous xls files of data about homes. The article has four other graphs.