Designing size lenses for aniseikonia correction usually involves considering only the shape factor of the total spectacle magnification. However, for minus prescriptions, an increase in the shape magnification of a lens does not always result in total lens magnification. As the front curve of a lens is steepened (which increases the shape magnification), the sagittal depth of the lens is increased, which effectively increases the vertex distance. For a minus lens, an increased vertex distance means decreased power magnification; depending upon the lens power, the decrease in power magnification can totally offset the increase in shape magnification. The shape factor-power factor relationship of high minus size lenses is presented, along with general guidelines for proper design of size lenses.