In some measures the size (a length dimension in the expression) can't be obtained, only calculated as a function of another dimensions and parameters. Illustrating below by the main cases.
;Weight-based (spheroidal) particle size: Weight-based particle size equals the diameter of the sphere that has the same weight as a given particle. Useful as hypothesis in centrifugation and decantation, or when the number of particles can be estimated (to obtain average particle's weight as sample weight divided by the number of particles in the sample).  This formula is only valid when all particles have the same density.
:where
::: diameter of representative sphere
::: weight of particle
::: density of particle
::: gravitational constant
;Aerodynamic particle size: Hydrodynamic or aerodynamic particle size equals the diameter of the sphere that has the same drag coefficient as a given particle. 
: Another complexity in defining particle size in a fluid medium appears for particles with sizes below a micrometre. When a particle becomes that small, the thickness of the interface layer becomes comparable with the particle size. As a result, the position of the particle surface becomes uncertain. There is a convention for placing this imaginary surface at a certain position suggested by Gibbs and presented in many books on interface and colloid science.