Hi, there are so many contributions to this topic which practically explains everything. You should search first before asking a question. Shortly, crystallite size, crystal size or grain size practically describe the same property. Crystallite possibly explains that the crystals described are unusually small. A grain is defined by a volume which has the same orientation. Engineering sciences consider the orientation by a number described by a minimal misorientation value which is MUCH bigger than crystallographers use. There already parts of a degree are sufficient enough to define another grain, crystal or boundary. This means, regarding grains you possibly have to check which misorientation angle is defined. Commonly, this is pure arbitrariness and mainly controlled by a property (elasticity, plasticity, conductivity etc.) which considerably changes (nothing changes that abrupt but they have to define one value as criterion). Default value is often 5° but it really depends on the respective property considered. This is far away from values derived from techniques like powder diffraction. Therefore, grains size and crystal size distribution can look different. 

Hi, there are so many contributions to this topic which practically explains everything. You should search first before asking a question. Shortly, crystallite size, crystal size or grain size practically describe the same property. Crystallite possibly explains that the crystals described are unusually small. A grain is defined by a volume which has the same orientation. Engineering sciences consider the orientation by a number described by a minimal misorientation value which is MUCH bigger than crystallographers use. There already parts of a degree are sufficient enough to define another grain, crystal or boundary. This means, regarding grains you possibly have to check which misorientation angle is defined. Commonly, this is pure arbitrariness and mainly controlled by a property (elasticity, plasticity, conductivity etc.) which considerably changes (nothing changes that abrupt but they have to define one value as criterion). Default value is often 5° but it really depends on the respective property considered. This is far away from values derived from techniques like powder diffraction. Therefore, grains size and crystal size distribution can look different. 

Please have a look at the below links.

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

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

Crystalline size usually measured by XRD, it is related to crystal size of a material while particle size is measured by laser diffraction or SEM, it is related to a particle containing many crystals