In an idealized situation larger crystals should have stronger scattering since each incoming photon has more chances to interact with the electrons (there are more electrons to interact with/longer interference cross-section).

In a somewhat more realistic approximation there’s a ’soft’ practical limit on the crystal size imposed by absorption , lattice effects, and mosaic spread. If we take an even more realistic view then there’s yet another limitat imposed by practicalities of crystal cryoprotection (very large protein crystals are difficult to freeze properly).

A.

I don’t have one explicitly, but the lecture made me think.

For example, Feynman discusses two issues that play a role in how photons are reflected:
1) the material
2) the depth of interaction

I wonder if a larger crystal of the same protein (and packing) will always reflect more photons. If photons are reflected based on probabilities in a peak and valley type of manner then perhaps reducing the crystal size will improve diffraction by moving from a valley to a peak.