Because of policy interest in the size distribution of farms, there is an interest in understanding the causes of changing farm sizes. This paper addresses an overlooked issue in the literature on the determinants of farm size, namely, the empirical specification of farm size. We examine 5 different size measures: acres operated, land and building value, cash receipts, cash receipts plus government payments, and a constructed measure of the rental value of farms. We graphically show the difference in trends in farm size using the various measures for the U.S. and selected states. We then discuss how the results of an analysis of the determinants of farm size depend on the farm size measure employed. The data set is a panel data set of 48 states from 1960 to 1996.