North American crib mattresses must be between 51 58 and 53 inches in length and 27 14 to 28 inches in breadth to be considered legal thickness can range from 4 to 6 inches. When it comes to crib mattress sizes and the safety of your baby you want to be sure that the mattress fits inside the crib snugly with as little space as possible between the two. And what is a crib called in other languages and what are the dimensions there.