Or just make your own magic item like a cloak of some kind that's got Enlarge Person, or has Expansion on it. That's what we did for our player with the War Hulk. The on thing you'll have to watch out for is if you're an intelligent character, you'll really want to reconsider that option because you lose all INT based skills, and gain no BAB. He was okay because of his being a merolith dragon (four claws, bite, two wings, horns, and a tail) and pounce, but other people wouldn't be so lucky.