AFM can be quite useful for measuring lateral dimensions and height of particles and it can go down to a few nm in lateral resolution (depending on the size and shape of your particles) and even subnanometer vertical  resolution.  STM can also help if your particles are electrically conductive.

You can use the XRD diffraction , i.e. Debye-Scherrer method.

Scherrer equation via XRD

Hi.

If you are working in solution a simple yet cheap approach is (ultra) high pressure liquid chromatography. Just run a set of molecular markers, better if NPs with known diameter; then start the run with your own sample. Good luck!

Any of them, XRD, AFM, or Micro-Raman. I Usually use a combination of them to corroborate the info.

The widely used method is DLS instrument used to measure the particle size for liquid sample.

Eventhough AFM mesurements are quite long and needs a good particle distribution on the substrate surface that oblige you to control the sampling, this is a very reliable method. You could also use DLS as already mentionned, or NTA, SAXS, FFF, mass spectrometer, BET ... but for my part, I have clearly a preference for direct method as AFM or SEM/TEM !!

If you are using solutions of nanoparticles, the easiest method is DLS

DLS is very well suited for monodisperse solutions of NPs but not really for polydisperse one

It may depend on the size of the nanoparticles: if they are large (>100nm) and the size distribution is rather narrow, diffraction methods can work very well, in particular if you have calibrated standards. If the particles are very small (

We tried to study various solutions nanoparticles including polydisperse using Microtrac zetatrac. In the case of polydisperse systems equipment finds several maxima distribution, if they are sufficiently far from each other. For example 10 and 50 nm.

Unfortunately and to my point of view, a bimodal population of 10 and 50 nm with a very narrow distribution is very far from what most of people have to measure in real life. It is true that a DLS can give good results for such a case, but what happens if you get a solution with a size distribution going from 10 nm to 200 nm. I'm not sure DLS can  do the job. We have to keep in mind that the signal detected by the DLS is dependant to the power 6 on the size of the particle. 10 nm particles are detected with 64 million less intensity than 200 nm particles (if my calculation are correct) !!! 

Other than SEM and TEM, the nanopore technology is also able to characterize the particle actual size and distribution. In addition, dynamic light scattering (DLS) works as well, but it only gives the averaged particle size and the size is hydrodynamic one instead of actual. 

If these material are cryatallines, you can used XRD (Sherrer's equation)

you can use particle size analyzer to accurate determination  and accumulate and differential distributions for Nano and micro particles

SMPS 

To have a good representation of the size distribution separating methods are the first choice. Beside ultracentrifugation (if out of reach) may use also analytical centrifugation with lower acceleration.

The powders m.b. attested by the specific surface S by the nitrogen adsorbtion method (BET method) with the preliminary activation of samples at 300°C. The measurement error - about 3%. The specific surface average (further simply called the average) diameter dBET of particles is determined by the formula dBET = 6/(Sγ), where γ is an average density of matherial computed with allowance made for the phase composition of the powder (zum bei spiel for the ZrO2 the tetragonal phase has 6.16 g/cm3; the monoclinic phase has 5.85 g/cm3), which is determined by an X ray phase analysis.

there are particle size analyzers based on optical measurements (e.g. the Zetasizer and Mastersizer3000 from Malvern), though you need to suspend them for the measurement