I also agree that high enlargement factor is the way to go to get big grain. Not even Rodinal gives that much grain; at least I don't see it as drastically more grainy than even Replenished Xtol, making 16x20 prints from 35mm Tri-X.
If you must use a developer to get really sharp grain - try FX37. It is way sharper than Rodinal, and gives honest gorgeous grain.
But cropping negatives gives infinitely more grain than any developer will. Shoot film as if it was intended to be cropped at printing time.
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