Lids of the same make may have different shape depending on range. See here
http://www.araisuperstore.com/Arai-Interiors_ASZV5.aspx so worth trying them all!
shark RR2 is more of a long oval (I wear a medium arai gt) and tried a medium but it wont even pass my skull!
Oh, you can always swap the cheek pads up/down size for perfect fit!