There are various analytical techniques such as FIB-SIM, SEM, EBSD, and TEM that can evaluate grain size.
Strictly speaking, the crystallite size measured in XRD and the grain size made in these observation methods are different, but here we will treat them as similar.
With XRD, the crystallite size can be evaluated using the fact that the full width at half maximum of a diffraction peak is wider as the crystallite size decreases.
However, there is a limit in the evaluation of the crystallite size using XRD Between a few nm to a few hundred nm.