Get the right size. âThe main thing with kidsâ bikes is having the right size bike for the rider,â says Jon Posner, store manager at Race Pace Bicycles in Ellicott City. Childrenâs bikes are measured by the diameter of the wheel, with sizes ranging from 12 inches to 24 inches. The appropriate size depends on the childâs age and height or leg length. For example, most 2-year-olds start on a 12-inch bike. To test proper fit, the American Academy of Pediatrics (AAP) recommends children sit on the seat with their hands on the handlebars.