Body size influences performance in many bee species and may be influenced by nesting cavity diameter in cavity-nesting bees. Megachile rotundata (Fabricius) (Hymenoptera: Megachilidae) is a commercially-managed, solitary cavity-nesting bee. In M. rotundata body size has low heritability and is strongly influenced by the size of the larval provision and the diameter of the nesting cavity. Commercial nesting boxes have cavities that are 7 mm in diameter. Our goal was to examine the effects that nesting cavity diameter has on M. rotundata body size and performance by manipulating the size of cavities that are available for nesting. We provided bees with nesting cavities that ranged in size from 4 to 9 millimeters in 1 mm increments. To assess body size we measured mass and intertegular span. To assess performance we measured wing area, wing loading, sex, overwintering survival, pollen ball occurrence, and diapause status in the offspring. We also examined the reproductive output from the different nest cavity diameters. We found that the 8 mm cavities reared bees with the largest mass, and 4 mm cavities reared bees with the smallest mass. We determined that the 7 mm nesting cavity is optimal for offspring yield, the 8 mm nesting cavity is optimal for performance, and the 5 mm nesting cavity may be optimal for conservation efforts of other cavity-nesting bees. Based on the desired outcome of the bee managers, nest sizes differing from the standard may provide an advantage.