[vonBaloney]

Do you have any general guideline of how much grain to add for a specific size, let's say, the common sizes of 4x6, 8x12, and 16x24?

Not really, it is image and print specific -- we just eyeball it, whatever looks good. We print out small test sections (so we don't burn too much paper on full prints) at 100% to test as needed (you'll see the grain more in some areas [like sky] than others). Trial and error mixed with experience, just like being back in the darkroom...