The AT4.0 Headliner is a replacement headliner for your AT4.0 helmet. The headliner follows the standard sizing so please order the size that correlates the to the size of your helmet.If you wish to change the size of your helmet you can order a different size of headliner to change the sizing. The AT4.0 helmet comes in three different shell sizes and if you wish, you can change its size simply by just changing the padding but only if the size you are changing to is inside the same shell size range.The shell size ranges are:XXS and XS share the same shell so the padding is cross compatible.SM has its own shell size and we offer special thicker padding for the SM here.ML, L/XL and XL/XXL are also all the same shell size so padding in cross compatible between these sizes. If you want to make your ML tighter we offer a special thicker ML padding here.