Dynamic Light Scattering, also known as Photon Correlation Spectroscopy, is a useful means of determing a particle's size. Shinning a monochromatic light source onto particles in Brownian motion causes a Doppler Shift to occur. This changes the wavelength of the incoming light. This change is related to the size of the particle. By measuring the diffusion coefficient of the particle and using the autocorrelation function (a decaying exponential) the sphere size distribution (SSD) can be determined as well as a description of the particle's motion in the medium.
To understand more about dynamic light scattering follow these links:
Instruments that use DLS
Uses for DLS
Papers on DLS