They get about 4 - 4.5 inches big. Not coral safe, but you could try adding some cheap ones to test. Just feed them crustaceans. I saw one of these puffers at my lfs
as well. It was all by itself since the shop keeper told me he was violent toward tankmates. They are very interesting fish. Depending on the personality you get, the spotted puffer might not accept tankmates. If you do buy it and it starts to constantly nip your coral or fish, then return it back to the lfs
Edit: this little guy only need 30gal and you can feed him frozen raw shrimp at the grocery store. Thaw before feeding.