Chitons are little tiny creatures that crawl all over the rockwork grazing for film algae. As you can see above, sized against a dental tool while crawling on a cutting board, these critters are small. This one was on the shell of a Maxima Clam. There are many kinds and colors, but their shape tends to remain the same. I have seen larger ones occasionally. Check your tank late at night and see how many you'll find; you'll be surprised. Reef-safe.
Usually these are about 3/8" long, but I've seen some in aquariums nearly the size of a thumb. This one below is a little one, just enlarged with a macro lens.
Below gives you a sense of perspective. It's on the side of a Hammerhead Algae Float cleaning magnet.