A standard crib mattress is 28 inches by 52 inches. At this size, many standard crib mattresses are able to change, along with the crib, into a bed for a slightly older child. These dimensions are in compliance with federal law regulations set by the United States Consumer Protection Safety Commission.
Some parents choose to purchase a mini crib instead. This type of mattress is only 24 inches wide and 38 inches long. This kind of mattress is available at different levels of thickness, which makes it ideal for travel and portability.
All crib mattresses, regardless of size, should be firm so that the baby's face doesn't get buried in the mattress when sleeping on his or her stomach.