The standard size for a crib mattress is 27 inches by 51 inches with a thickness not exceeding 6 inches. Mini crib mattresses are typically 24 inches by 38 inches. Its important to measure the interior dimensions of your crib to ensure that the mattress fits properly and meets safety standards. The mattress should fit snugly inside the crib, leaving no more than a 1-inch gap between the mattress and the crib wall.