A standard size for a crib blanket is 36 inches wide by 54 inches long, as a crib mattress is usually 28 inches wide by 52 inches long. Some precut batting for crib quilts is sold as 45 inches wide by 60 inches long.
The extra width and length added on to the crib mattress size is to allow for drape. Some commercial blankets are smaller, for instance, at about 35 inches by 45 inches. Whether a smaller or larger blanket is used depends on the parents. For example, a larger blanket covers more of the crib, but a smaller blanket is easier to pick up with the baby.