If you can't or don't want to pay for ob., then you may want to try the Sporn No Pull Harness. Both of mine have been to ob. but they're still pullers and I don't want damaged windpipes, so we use the harnesses. They're fantastic. As to what size, take Katie with you so that you can try the different sizes on her. Chester and Millie both wear a medium. Chester's only about 12 lbs, but he's very deep chested, so we went with the medium.