The Crib mattress size is 51 5/8 inches in length and 28 inches wide. These are Crib size bed measurements. Crib Size Mattress depths can vary from 4 inches to 6 inches, consult your crib manual. IMPORTANT: Some cribs may vary in width and length and may be a potential safety hazard if the mattress does not fit snugly. A mattress that doesn't fit properly might allow your infant to crawl beneath it and that would be DANGEROUS. Requirements for a full-size crib have been published in Code of Federal Regulations in Title 16, Part 1508 by the US Consumer Product Safety Commission.