Toddler Mattress Size is 51 5/8 inches in length and 27 inches wide. These are general toddler size bed measurements. Toddler mattresses are also generally Baby Crib mattresses. Mattress depths can vary. IMPORTANT: Some toddler beds may vary in width and length and may be a potential safety hazard if the mattress does not fit snugly. A mattress that does not fit properly might allow your youngster to crawl beneath it and that would be DANGEROUS.