To choose the size of crystal for diffraction, the following factors should be considered:

1 X-ray beam size. 

in general, size of crystal < size of the X-ray beam

2 x = 1/u (linear absorption coefficient)

this is espeically imfortant for compound with u larger than, say, 10 mm-1.

for example, the compound containing I have u = 30 mm-1.

x = 0.03 mm

3 crystal shape

the crystal shape should be isotropic, like sphere and cubic.