I've made most at 36 by 48 inches. I have also done a 36" by 36".
All sizes have thier uses, with baby quilts. A smaller one is easier to carry around and put down for the baby to play on the floor, but it is outgrown faster. Having said that, the 36" by 36" one I made for my son he is using at age 2 1/2 on his bed. He gets hot easily and it's easy for him to kick off, but big enough to cover him up.