Abstract A representative subset of a stratified random sample of LACIE (Large Area Crop Inventory Experiment) segments which are 5 nmi x6 nmi in size were ground truthed and were used to derive field size, length, and width distributions for winter wheat, spring wheat, corn, soybeans, water, and “all crops” for areas in nine states in the U. S. Great Plains and one state in the Corn Belt. Field sizes for spring wheat and soybeans appeared log-normally distributed whereas the other crops and “all crops” did not fit the log-normal distribution well. The modal field size was near 10 acres for most crops studied. Winter wheat, spring wheat, and corn were found to have field width modes near 90 m and soybeans had a mode at 200 m. About 25% of all fields were found to be more narrow than 100 m. Field length modes were found at 400, 800, and 1600 m (I mi) due to the section line road system in the agricultural midwest and the homesteading of 160-acre farms (800 m x 800 m). Based on these field size distributions and a simple theoretical model it was estimated that fields of corn, soybeans, winter wheat, and spring wheat have Landsat MSS pixels which are on the average 40% pure (i.e., 40% of all pixels contain a single generic class), and that this will increase to 75% at the thematic mapper resolution.