 Solution

For each spacing pair, the largest geometrical period occurs with parallel projected reciprocal vectors. The closest pair is $d_{\mathrm{Cu}}=0.208\mathrm{nm}$ and $d_{\mathrm{substrate}}=0.212\mathrm{nm}$, givingHowever, the crystal is only $10\mathrm{nm}$ across. It contains at most $10/11.024\simeq 0.91$ of this period, so the largest formal period cannot appear as a repeated fringe train inside one crystal. It can instead give a broad variation of contrast. A period of order the particle diameter is the largest directly sampled scale, and even that is poorly constrained by one particle.

The distinction matters if “see” means a complete repeat. Of the exactly parallel spacing pairs, the largest other period is approximately $2.09\mathrm{nm}$ from the $0.208$ and $0.231\mathrm{nm}$ pair. If relative rotation is continuously variable, the closest pair can instead be rotated slightly to shorten its period below $10\mathrm{nm}$, so $2.09\mathrm{nm}$ is not a universal maximum visible period for arbitrary orientations.