Burying beetles (Nicrophorus investigator: Coleoptera) reproduce on small vertebrate carcasses and are well-known for their extensive bi-parental care of larvae within an underground brood chamber. The carcass is the only larval food source. In this study I demonstrate that the relationship between the number and size of larvae is inversely correlated, that parents can measure the quantity of food available to offspring and can adjust brood size (and thus larval size) through egg laying patterns and selective culling of larvae, and that adult size at emergence the following summer is highly correlated with larval size at the end of the larval feeding period. I generated a fitness model for optimal offspring size using measures of brood size and the relationship between larval size and over-wintering success. The model produces an optimal offspring size similar to that observed in a wild population of Nicrophorus investigator.