Its better that the helmet is a little to big than to small. Once you resin and fiber glass it, it will not be as flexible as it is without resin which means it might be alot harder to get on and off. That also means you might have to trim the helmet to get it to fit right. I found all of that out the hard way.
But if you scale the helmet a little on the big side (half a CM or so) then it will allow for some wiggle room that can be fixed with padding.
Looks great over all though!