Body size strongly predicts fecundity and flight performance. In solitary bees, the mother controls the amount of food available for larval growth through provisioning. The mother also determines the sex of offspring and the environmental conditions of larval growth by choosing a nest site. In Megachilid bees, heritability of body size is low. Therefore, mothers may be able to determine the body size of offspring and make life history choices accordingly that maximize fitness. However, the amount of variation in body size determined by maternal nesting behaviors has not been well established. We measured provision size, position of the brood cell in the nest, diapause status, temperature during the larval stage, sex, adult body weight and intertegular span in field-collected nests of Megachile rotundata. Mothers gave diapausing offspring, female offspring, and those laid first in the nest larger provisions. Provision size alone predicted 18% of the variation in adult body size. Adding other significant predictors of body size including sex, diapause status, and position in the nest increased the R2 to 48%. Temperature of the nest during the larval stage did not affect adult body size even though some nests spent multiple hours above 40C, which is known to be a stressful temperature. The size of provision did not affect survival. Our results suggest that M. rotundata mothers are able to determine about half the variation in the adult body size of their offspring through provisioning and nest building behaviors.