Thre are really no standards sizes Depends on the age of the baby and if you want something for newborns or one that the child will grown into. Usual to start is about 36"x45" and then 45" x60" for one a bit larger that fits most crib beds. I usually say " large enough to cover the kid" but not "too large for the bed"
Standard batting sizes can also control the size. Most packaged battings are 45"x60" but Warm Co makes on that is 32"x43" or so.