First, we must first have a reasonable method to determine the crystal size distribution and make sure that they have indeed changed

Second, this is related to the preparation or processing method. If only a part of Fe is doped by the ion exchange method, the crystal size change should be ignorable.

Third, if Fe precursors are added in the hydrothermal process (if the hydrothermal method is used), the size of the molecular sieve will be smaller compared to that without Fe precursors insome cases, which requires detailed research.

I can describe a crystal size by SEM or MET analysis

you can perform the powder XRD of zeolite and Fe modified zeolite samples and try to find out the average crystalline size by using Scherrer equation. Hope it will give you some change. The change in crystalline size also depends upon the type of modification you have done i.e. doping or composites.