We make these little "chew" blankets at the quilt shop where I work. They're 30" square, cotton on one side, and Minky on the other, with no batting. Then you quilt them -- usually just diagonal lines, every 4th row of bumps on the Minky. And add a binding. Those are just right for drag around blankies.
Receiving blankets are 36" square for newborn, and 45" square for older babies.
Or you could make a knit or double-gauze swaddling blanket. The new moms love those, and they are usually 36" to 45" square.
Wrap-up quilts are usually about 42" square; or 42" X 54" if you want something bigger; or 42" X 60" for a crib or toddler quilt.