This paper describes a study to determine the most appropriate particle size parameter for polydisperse powders dispersed as aerosol and detected by light scattering. It is relevant to many areas of engineering research. By means of calculations of the intensity of light scattered from particles of different sizes, it has been shown that the surface median aerodynamic diameter (SMAD) is an appropriate measure for polydisperse aerosolised powders. The microscope-based procedure for obtaining the SMAD is described, as well as its implementation for narrowly-graded polydisperse powders of fused alumina of the type which have been widely used in studies of particle transport in wind tunnels, It was shown that, for the fused alumina powders examined, SMAD = 1.25 x (PASMAD), where PASMAD is the projected area surface median aerodynamic diameter.