What's the correct way to garnish with greens?
The key is how you expect the person to eat it.

Some garnishes are not for eating at all - the sprig of mint or entire stalk of rosemary that the eater is expected to just remove and set aside. I don't do this - and plenty of judges on plenty of cooking shows have intoned "never put anything on the plate that I am not supposed to eat" - but if you do, make it as large as possible and just one piece so it's easy to remove.

Assuming I'm supposed to eat it, am I supposed to get a little in each bite? Cut it up small. Am I supposed to get rather a lot in each bite? Leave it bigger. Is it a herb that tastes different chopped than whole? If so, which way do you want it to taste?

If the item is picked up to eat (eg an hors d'oeuvre) then typically you leave the pieces larger so they're less likely to cascade off. Search for images of chive garnish and you'll see tons of teeny little pieces on a tomato salad, and single or double 2" pieces draped across a devilled egg.

Above all, practice and confidence. If you know why you're putting parsley, mint, cilantro or chives on the plate at the last minute, you'll also know what size, how many, and whether they are in one little pile or all over. And when you know all that you'll bring out the plate with confidence.