In the development of an interaction potential for polyethylene, the effects of attractions and excluded volume are often difficult to separate. Here we consider an experimental quantity which is dominated by the excluded volume (the compressibility) and require that a hard-site model of polyethylene predict the experimental compressibility. The site diameter in the model is varied until this requirement is met. We use a rotational isomeric state model of polyethylene, and the compressibility of the model is calculated with polymer reference interaction site model (PRISM) theory. The experimental compressibility is found from the Olabisi-Simha equation of state. The effects of state variables and chain length on the site diameter are investigated.