Well, Idk I got a medium since its for my wife's head. Hers is considerably smaller than mine so I guesstimated. I used to play football and I wore an extra large helmet so that's what I got for me and I just guessed she would be good for two sizes smaller than mine and I happened to hit gold. I have no more advise to give you than that man, I hope it helps.
I used to play football too and i had a large helmet so i guess i need large size !
its really awful that the stores in austria ( linz) dont sell the padding itself-.-'