The two "baby" quilts I've made have been about 36 x 45. I plan on doing my "drag around" blankets this size for a few reasons... I can usually get queen-sized batting at a good price, and I can cut one into six pieces for this size quilt. It's an easy size to carry around and lay on/under for most little kids. It's also a good size for kid picnics, tea parties, etc. And it's a good size for using in the car.
Another bonus-- I can buy one yard for my backing fabric, and just adjust the length to the width of that fabric.