Round Crib Mattress Dimensions are generally 42 inch diameter. These are fairly standard round crib mattress bed measurements. Round Crib Mattress depths will vary, five to six inches seems very common, dependent upon safety considerations. Some round cribs may vary diameter 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.