my only pointer for you so far would be to hold off posting wip pics untill you have more actual sculpting done, so far there isnt enough to see what you're doing, it could be anything.
I molded a storm trooper helmet on one of those styrofoam heads (actually it was a fiberglass copy of a styrofoam head I made as practice). The one I've got looks slightly larger than yours and the helmet was too small for my head. I don't have a particularly large head. I ended up molding a second helmet over my original to get enough space to fit my head in it. I would recommend that you take some measurements of your head and compare them to the model you have. Don't forget to leave some extra space so you can get it on and off.
I suspect that the styrofoam heads are intentionally sized slightly small to make the products they are advertizing look larger.
