Properly designed and geometrically similar small-scale models can provide insight into component and system performance of large vehicles for a variety of engineering disciplines for a fraction of the cost compared to full scale fabrication and testing. Using the density, specific heat, magnetic permeability of the fluid, the rotation rate, and the size, the author provides non-dimensional performance properties broadly encompassing the hydrodynamic, acoustic, electro-magnetic, thermal, and structural fields. The preferred numbers based on a geometric series as devised by Renard are introduced. The author then provides a method for defining a mean and standard deviation of a size differences from a standard geometric series. In particular, statistics for customary and metric material specifications of sheet metal, square and round stock, electrical wire, fasteners, beams, and pipes are quantified. The study concludes that preferred numbers are best for standard material sizes and scales and suggests potential procedures for improving systems engineering.