Particle size is one of the most fundamental measurements for the particle technologist, however, unlike in the macro world where "size" is easy to understand, in the micro world "particle size" is less easy to define, and even harder to measure.
Particles are rarely simple geometric shapes, and describing an irregular shape and converting this into a ‘size’ is a challenging task.
There is no single definition of particle size as it is dependent upon the measurement technique used, for example microscopy uses area projection, electroresistance uses particle conductivity, sedimentation uses hydrodynamic particle size and laser diffraction uses the optical properties of the material being measured.
Remember that a particle's shape (or sphericity) is independent of its size. Sphericity is the ratio of the surface area of a sphere (with the same volume as the given particle) to the surface area of the particle. Depending on the application the particle size as well as its shape may be import, e.g. spherical particles are needed for toner powders whereas polishing powders require sharp edges to produce an abrasive action.