Crystallite size is the smallest - most likely single crystal in powder form. The crystallite size commonly determined by XRD.

Grain is either a single crystalline or polycrystalline material, and is present either in bulk or thin film form. During the processing, smaller crystallites come closer and grow to become larger due to kinetics. Therefore, in the most likely scenario, the grain is larger than a crystallite. And, the grain morphology is commonly determined by SEM (but not XRD).

Particle may be present as a single crystal or an agglomeration of several crystals. Therefore, particle is under no circumstances smaller than crystallite size. In the ultrafine nano regime, particle size and crystallite size may be the same. XRD and TEM are commonly employed to ensure that there is any difference between the crystallite size and particle size.

Regarding the size comparison between grains and particles, it is however difficult to say whether the grain is bigger or particle is bigger. Sometimes, particles are evolved from controlled agglomeration of small grains, and alternately grains may also be grown from smaller particles by annealing at a higher temperature.

Further, since the crystallite size and grain size depend on nucleation and growth kinetics during processing, they may be different for different materials because the temperature sensitivity and melting points are also different for different materials.

Please, have a look at this previous post:

https://www.researchgate.net/post/What_is_the_difference_between_crystallite_size_grain_size_and_particle_size

Or, to make it short: No!

A grain may consist of several crystallites. Depends on the material.

Summarizing:

Crystallite size: Size of a single crystal.

Grain size: Size of a particle. In the specific phase of a particle formed by only one crystal, both terms are the same. 

By other hand, when you calculate the crystallite size by WH-Plot or Scherrer , you are determined the "diffraction coherent domain", not the grain size. 

To estimate the grain size, you have to do Particle size distribution by LASER, TEM, SEM, etc.

Crystallite size is the smallest - most likely single crystal in powder form. The crystallite size commonly determined by XRD.

Grain is either a single crystalline or polycrystalline material, and is present either in bulk or thin film form. During the processing, smaller crystallites come closer and grow to become larger due to kinetics. Therefore, in the most likely scenario, the grain is larger than a crystallite. And, the grain morphology is commonly determined by SEM (but not XRD).

Particle may be present as a single crystal or an agglomeration of several crystals. Therefore, particle is under no circumstances smaller than crystallite size. In the ultrafine nano regime, particle size and crystallite size may be the same. XRD and TEM are commonly employed to ensure that there is any difference between the crystallite size and particle size.

Regarding the size comparison between grains and particles, it is however difficult to say whether the grain is bigger or particle is bigger. Sometimes, particles are evolved from controlled agglomeration of small grains, and alternately grains may also be grown from smaller particles by annealing at a higher temperature.

Further, since the crystallite size and grain size depend on nucleation and growth kinetics during processing, they may be different for different materials because the temperature sensitivity and melting points are also different for different materials.

In addition, note that the direct application of the Debye-Sherrer formula gives the crystallite size, not the grain size

I would also like to direct you to one of the previous posting here in research gate posted by somebody else. Hope this will add more insights into particle size vs crystallite size to resolve your questions.

https://www.researchgate.net/post/Can_someone_suggest_which_is_the_more_accurate_way_to_calculate_the_particle_sizes_SEM_TEM_XRD

Dear colleague Ahmad,

Crystallite Size is Different than Particle Size. A particle may be made up of several different crystallites or just one crystallite so in this case (particle size = crystallite size)

Crystallite size often matches grain size, but there are exceptions

Crystallites are coherent diffraction domains in X-ray diffraction.

Particles are chunks/pieces (usually very small, below 1 mm) of solid matter, ensembles of atoms. Particles can be as small as two atoms (the nitrogen particle for example, N2)

Grains are volumes, inside crystalline materials, with a specific orientation.

Particles can be polycrystalline, single crystal or amorphous. A 100 nanometer particle of gold, for instance, can be made of:

a single gold crystal,

many grains with a grain size

i am quite agree with Claus Moseke

Crystallite Size is Different than Particle Size. A particle may be made up of several different crystallites or just one crystallite so in this case (particle size = crystallite size) Particle or grain which consist of several small crystallite may have different orientation as determined by XRD diffractrogram peks present.