you can try with X-ray techniques. Depending on your size range and dilution, small angle X-ray scattering (1D or 2D) and/or X-ray diffraction can give you some information on this (if you are lucky you get also some size distribution).
Sir if my knowledge about this is right, its somewhat easy. You first take image analysis(TEM/ SEM). Then you measure the diameter and thickness of particle and just take the division of that. May be right, you go through the available papers in site.