By adjusting the sputtering rate and gas pressure, it is possible to form nanoparticles of different sizes, phases, and materials. We have investigated the spatial distribution of sputtered particle formation using a vertical, linear arrangement of substrates. Collecting the particles soon after they are formed, before they have time to grow and agglomerate, allows one to obtain a narrow size distribution. In the case of molybdenum, a narrow distribution of cubic particles is formed at relatively large distances (8 cm) from the source. These cubic particles collide and self-assemble in the vapor into arrays of larger cubic particles. The particle size histograms are fitted to lognormal distribution functions. How supersaturation occurs is discussed qualitatively as a function of the distance from the substrate, sputtering rate, and the mean free path in the vapor. This method of nanocrystalline particle formation has potential use in magnetic and opto-electronic (quantum dot) applications, where a narrow size distribution is required.