It fits just like a normal beanie underneath, maybe a little snugger. I have the ninja mask and a size large RED trace and it fits real good. With a normal beanie and gog's underneath is isn't quite as snug as I would want it but it is perfect with the ninja mask. Go for it.
Bonzai_Warrior "haha holy shit dood...shes def. showing how many fingers she uses to play with herself"