I too suffer from having a huge head ( 7 7/8 - 8 ). I can't even but a ski helmet, my friends love to hand me a xxl in a ski shop and laugh like little girls when it won't even come close.
I currently have a head tip polo helmet, it is great, and I believe it was a xxl. I got in touch with them through one of their reps and got a deal on it back in my guiding days. They should be able to get you some dimensions of their standard size helmets on their website.
Hope you find something that works, I feel your pain. I could never bring myself to buy a helmet that almost fit and shave it down. I was always worried about degrading it's protection, and ruining a $125 helmet and having it still not fit.