If a One K Helmet does not come in your size, you can easily change the liner for a perfect fit. Our liners also provide ultimate flexibility for growing children, shared helmets, or women who wear hair differently for schooling and show! Two shells make up all sizes. The base shells for the Furlong are Medium and Large. Use the Medium size helmet with a Small or X-Small liner to change the fit of the Medium helmet into a Small or X-Small. Use the Large size helmet with an X-Large or XX-Large liner to change the fit to Xl-Large or XX-Large.