dont know the exact size but I wear like a 7 1/2 hat and it fits me fine. The helmet comes in two half's. the face plate and the back plate so there is no problem putting it on like a stormtrooper helmet.
Here are the pics of the helmet / mask. It is made of fiberglass and is real solid. Would be will to sell LMK. The back plate is held on by strong magnets (I forget what they are called, I think rare earth magnets). There were three sets but one has fallen off but should be no problem getting another pair.
A guy at a paintball scenario I played in did a pretty good Doom on the cheap.
When I say "pretty good" I mean "It looked cool until he got shot all to hell" LOL
That's me in the Cap suit. It was a Marvel comics based scenario. In addition to Cap and Dr. Doom we also had Mephisto, Drax the Destroyer, Surtur, Adam Warlock, Nick Fury, Thanos, Morg the Merciless, Punisher and Spider-Man.
Here are the pictures of me wearing the helmet and one of the inside. The helmet is signed by the maker and dated "2007 #3" (cast). I cant remember the guys name but he did post the sculpt here on the RPF.