The cattle industry is very competitive which is forcing cow-calf producers to strive for efficiency. Research has shown that as a cows mature weight increases, feed efficiency decreases, as well as reproductive efficiency and other production factors. The purpose of this paper is to (1) identify the economically optimum cow size when charging for grazing public lands on a true Animal Unit Equivalent (AUE) basis and (2) determine if the current practice of charging on a per head basis for grazing public lands has an effect on the optimal cow size. To simplify the complexity of this problem, three different resource bases common in the Intermountain West (resource base 1, time grazing = 100%; resource base 2, time grazing = 75%; and resource base 3 time grazing = 50%) were defined, as well as three different weights of cattle (small, medium and large). Grazing plans were created for each resource base and winter rations were balanced to ensure adequate nutrition and accurate budgeting. Linear programming was used to determine an optimal cow size for each resource base when charging on a per head basis and by a true AUE. When grazing on public land was charged on a true AUE basis, the small cows generated the highest net returns on all resources. Also, each resource base was able to maintain a larger number of the smaller cows than the medium or large cows under these conditions. When grazing on public lands was charge on a per head basis, as is typical, the large cow generated the greatest net returns on resource base 1 and 2. However, the small cow generated the greatest net return on resource base 3. These findings suggest that the current practice of charging for grazing public land on a per head basis does have an impact on cow size.