The standard toddler mattress size is the same as a crib mattress, which is 27 ¼ inches x 51 ⅝ inches. Many cribs, often called convertible cribs, turn into toddler beds using the same mattress. Some non-standard toddler beds range from 27 ¼ - 31 inches wide and 51 ⅝ - 71 inches long, and a corresponding mattress should come with the crib. It is important to use the mattress that comes with the bed to ensure that it fits properly, and you shouldnt have more than an inch or two-fingers width between the mattress and sides at any point. If you are upgrading to a more extensive "big kid" bed, such as a twin, the mattress will be considerably larger.