I am engineering nanocages and it will be approx. 3.1nm in diameter and approx. 15.3nm in length. My colleague tried using TEM but it is at its limit. I am open to suggestion.
If you very well disperse your nanoparticles over HOPG (highly-oriented pyrolytic graphite) which is atomically smooth, you can have precise size measurement from average step height (ASH) using AFM. Another option is to use polished Si wafer to disperse your nanoparticles for AFM studies.