A method and system of measuring the size distribution of particles within dilute colloids, for example, through variation of the minimum detected size of aerosolized colloid particles. The method of determining the size distribution of particles in a fluid, involves forming a stream of aerosol droplets of the fluid, the droplets containing particles and dissolved material, evaporating the droplets to generate particles, and measuring the concentration of particles by varying a detection threshold. A system or apparatus for determining the size distribution of particles in a fluid, includes a droplet former for forming a stream of aerosol droplets of the fluid, the droplets containing particles and dissolved material, and a condensation particle detector for evaporating the droplets to generate particles and for measuring the concentration of particles, the condensation particle detector having a variable detection threshold.