Lee, I thought I'd chime in here, since my ears started burning...I just measured my P2 bio, and it pretty much agrees with the measurements that Gene took for you. I always try and measure the dome at the widest, which is a little higher up than where Gene took his, and it comes out to 11-7/8", or just over 30cm. I was guessing the length from memory, and it is more like 19", as Gene showed.
Now these two pics seem to show that the hero bio at least, should be a bit wider than the mask, and reach down and partially cover the first set of dread holes.
I plan on sculpting a hero version from scratch, and I was planning on increasing the size of the stunt by roughly 15%. This would make it 13.8" wide, and 22" long. I may not actually go that large, but certainly it will be bigger than the stunt.
Now, since I HAVE one of your open mouth P2s, I thought I'd try and sit my bio on it. It doesn't work so well, to be quite honest. The bio isn't nearly long enough, though it is just barely wide enough (I can just about cram everything into the bio, but the back of the bio still doesn't quite overlap the top of the head). I'd guess that the stunt head was both narrower and shorter than you did your mask, to retain the right proportions.
You know I want one of these, so if you want to consult on anything, seeing as how I have your mask and my bio handy, just ask.