Kiddimoto Kids Helmet - Neon Orange Medium Eleven little vents keep your childs head cool. A rear adjustment wheel, internal comfort padding, and adjustable straps help to ensure a perfect fit. Made from lightweight ABS hard plastic. Available in two sizes: Small (18.89 inches - 20.86 inches) and Medium (20.86 inches - 22.83 inches). To choose the correct size, measure around the widest part of your child's head, about an inch above the eyebrows.