Have you tried the BICO cardigan? It's written for a bulky weight yarn, to fit a 6 mo old. If you switch to crochet cotton (or even a lace weight yarn) and a hook the right size for it, I'd be willing to bet it'd come out about the right size for a teddy bear. It's also easy enough to adjust the size after you've made the first one, and understand how the measurements all work together.
I've also seen another child's sweater, crocheted in two T shapes, then stitched together along the side seams and across the tops of the shoulders. That gives you the ability to make the hole opening any size you need. Once you figure it out, you can even make it in one piece instead of two.