The goal of this work was to develop a simple technique for sizing colloidal particles by means of electrospray and aerosol techniques. Size distribution of different types of colloids (oxides, metals, and polymers) such as silica, gold, palladium, and polystyrene latex particles, with different nominal sizes below 100 nm was determined online. Nanometer-sized particles were dispersed into the gas phase as an aerosol via electrosprays operating in the cone-jet mode of a colloidal solution followed by a charge reduction of the sprayed droplets to unity and subsequent evaporation of the solvent. The size distribution of the generated aerosol particles was then determined by a differential mobility analyzer combined with a condensation nucleus/particle counter. For comparison, particle sizes were determined by electron microscopy (EM) using the samples which were obtained by (i) naturally dried sols and (ii) on-line deposited on a substrate during electrospraying. The proposed technique is capable of detecting the degree of dispersity of all colloid samples, and the measured values were comparable to results obtained by EM and dynamic light scattering. The results clearly show that the method described here constitutes a convenient, reliable, and rapid tool for the size determination of colloidal nanparticles.