IT depend on techniques from which we are synthesizing. It is purely depending upon methodology. for example i thermal vapour deposition it purely depend on temperature and time. Greater the time greater will be size. While in sol gel it depends upon pressure and temperature. In sputtering it depend upon at what speed particles are scattered.

Size of the nanoparticles depend on several factors. Synthesis procedure, reaction parameters, post-synthesis treatment and also, characterization techniques. Reaction parameters have crucial role in size and shape of NPs. Temperature, pressure, pH, concentration, time, solvent, precursor etc. influence the growth of NPs and also their aggregations. Post synthesis treatment like annealing, coating, functionalization etc. can modify further the size and shape. Utilizing these techniques the phase, surface, crystallinity, functionality can be altered. Finally, size or shape depends on the characterization techniques. Microscopic imaging directly provides size and shape of the image. Scattering technique provides statistical distribution of shape and size of NPs. Spectroscopy provides indirect measurement via calibration.