Using demographic data, a series of BOMAB phantoms were developed to study the effect of size, at a fixed height, of a BOMAB phantom using Monte Carlo simulations in three different whole body counting systems: the HML's scanning detector whole body counter, a FastScan whole body counter, and a StandFast whole body counter. The latter has had two counting geometries simulated—one for the recommended position, and another simulating a constant detector-to-front-of-phantom distance. The six phantom sizes corresponded to the following masses: 48 kg, 73 kg, 98 kg, 123 kg, 148 kg, and 173 kg. The effect of size varies with photon energy, as might be expected, and at any given energy is an exponential function of the mass. An equation has been found that fits most cases very well and is still good in poorer cases. Persons lighter in mass than the normal calibration phantom (73 kg) will have their body burdens overestimated by as much as a factor of 1.3, depending on mass, photon energy, and counting geometry. Conversely, heavier individuals will have their body burdens underestimated by as much as a factor of 1.9, depending on mass, photon energy, and counting geometry.