This report documents the field distortion of the Elionix ELS-7500EX electron beam lithography system at the University of Pennsylvania Singh Center for Nanotechnology at the Quattrone Nanofabrication Facility. The system is equipped with a 20MHz fixed clock and fixed focus. The aim of the work is to understand optimal field sizes to use for critical dimensions 80nm and above. Field uniformity was analyzed as a function of critical dimension and objective lens aperture (OLA) a.k.a. final paerture. As features scale down below 300nm, they are more susceptible to the systematic effects of field distortion. Suggested field sizes depend on the feature size that is desired.