A large-scale unitary-study for generic topologies of surface mounted permanent magnet brushless motors operated from sine-wave current regulated drives is described. A wide range of power ratings, from 1 kW to 1 MW, and a large spread of physical dimensions, with a stator inner (air-gap) diameter as low as 50mm and as high as 500mm, are considered. A parametric motor model employing eight independent variables is introduced. A sensitivity analysis based on the Response Surface (RS) method is performed and a Differential Evolution (DE) algorithm is applied to find the optimal solutions for different power ratings. The study provides rules for scaling the machine main design parameters with the power rating in order to achieve minimum active material cost per point of efficiency.