Cradle Mattress Size Dimensions are generally 18 inches by 36 inches. Additional cradle dimensions can be 18 inches by 35 inches, 15 inches by 33 inches, 17 inches by 31 inches and 12 inches by 28 inches, etc., depending upon the manufacturer. These are fairly standard cradle bed measurements. Cradle Mattress depths can vary, usually 1 to three inches thick, Of course, some manufacturers may build to their own size. Extreme care must be observed with Cradle mattresses and bedding.