A tub too big will stress him out. A tub too small, he will outgrow within months. If you are buying or building a rack, I suggest go with adult tubs and space some of it off to make the tubs smaller like use a 40qt tub and block half of it off. If not you can build a rack that can hold different size tubs like 28qt and the 40qts. If you do plan to get more BPs, for young ones, i suggest 6qts then move up to 28s.