Female blow flies (Diptera: Calliphoridae) demonstrate site preferences when ovipositing on carrion. Preference for oviposition sites is believed to reflect the suitability of the site such that the most preferred locations provide offspring with the greatest chance of survival. A number of factors might influence female choice, including area available for oviposition. For species known to aggregate during oviposition, the surface area of oviposition sites may be a key factor in the decision-making process. Using ImageJ, the surface area or volume of eight common oviposition sites on 13 fetal pig (Sus scrofa domesticus L.) carcasses was measured. Regression analysis was used to determine if body mass could predict the area or volume available for oviposition. We observed significant positive relationships between size of oviposition sites and body mass for all eight oviposition sites. The strongest relationships were observed for the face (R² = 0.86) and legs (R² = 0.84). To test the predictive capacity of this relationship, a blind validation study was performed. Five pigs of various body masses were randomly selected, and the available space of the eight sites was estimated using the regression equations. The validation study confirmed that the regression equations for most oviposition sites demonstrated their predictive capacity based on 95% prediction intervals. This research provides a tool that can be used to correct for differences in oviposition site area or volume between carcasses of varying mass so that researchers may gain a greater understanding of the oviposition preferences and aggregation behaviors in future studies.