Commercial agriculture in the United States is comprised of several hundred thousand farms, and these farms continue to become larger and fewer. The size of commercial farms is sometimes best-measured by sales, in other cases by acreage, and in still other cases by quantity produced of specific commodities, but for many commodities, size has doubled and doubled again in a generation. This article summarizes the economics of commercial agriculture in the United States, focusing on growth in farm size and other changes in size distribution in recent decades. I also consider the relationships between farm size distributions and farm productivity growth and farm subsidy policy.