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 Defender Pro are Large and X-Large. Use the Large size helmet with a Medium, Small, or X-Small liner to change the fit of the Large helmet into a Medium, Small, or X-Small. Use the X-Large size helmet with an XX-Large liner to change the fit to XX-Large.