Understanding the regulation of body size has important consequences for our understanding of life history trade offs, reproductive fitness and survivability. Recent studies in insects have begun to identify critical regulators of size. Insects undergoing complete metamorphosis grow in size as larvae but stop feeding and begin metamorphosis once a particular size has been reached. Since growth ceases as they approach metamorphosis, the growth rate and duration of an insect’s larval stages determines its adult body size. The complex process of size control involves multiple factors including nutritional availability, and physiological and genetic regulators. Previous research has described the cascade of endocrine events that cause larvae to cease growth. Although the key hormones involved in this process have been identified, the extent to which the mechanisms of growth are conserved across species remains unknown. Our study aims to develop a mathematical model to illustrate the roles of various factors on the final body size of insects across species. We utilize differential equations to reflect the relationships between growth regulators and compare the model to growth regulation in several insect species, including the tobacco hornworm, Manduca sexta, and the fruit fly, Drosophila melanogaster. The model depicts the production and suppression of these regulators during larval growth to visualize different scenarios that affect growth rates and growth cessation. By manipulating a variety of constants that can mimic normal and mutant insects of different species, the model predicts changes in metamorphic timing and resulting adult body size. Through our model, we seek to present a more comprehensive image of size determination and illuminate missing puzzle pieces in our understanding of growth regulation.