I made my first one 30" by 40" I think, and I've done some 36" by 48", and several about 36 by 36.
I don't think babies outgrow quilts so fast. My son is three and still uses the 36" by 36" one I gave him. Granted, in the winter it isn't enough, but right now, it's the only thing on his bed becuase it's more of a comfort item. But of course I made it for him, and I'm a quilter. So maybe non-quilting mamas would see it as "outgrown" already.
I do like to make baby quilts that are bright colors, so when they are older they can still have it across the foot of their bed, and it won't look like a "baby" quilt.