Particle size is a dimension which is determined by its geometry. For spherical particles, it is their diameter. For irregularly shaped particle it is the diameter of the equivalent sphere which has the same volume as a particle.
Silica particles are always obtained with a particle size distribution. Particle diameter for any HPLC packing is a mean value of all particle diameters. Most LC packing materials have a Gaussian type of particle size distribution, so the position of the maximum is the mean particle diameter, and the standard deviation represent the distribution width.
In HPLC practice, the less the particle size distribution the more uniform packing in the chromatographic column can be achieved, and the more efficient the column will be. The presence of the larger sized particles only effect the column efficiency, but the presence of the very small particles (fine), less than 1 Ám, may lead to the column frit clogging and rising of the column backpressure.
The effect of the particle size on the column efficiency and Van Deemter curve profile are shown in the figure below.
Van Deemter curves for the columns packed with particles of different size.