Luckily I was able to try mine on before I bought it. You might want to call in when you order. That way you can ask the customer service person how the sizing is done. They'll probably ask you to take a measurement of your dome, so you might want to get a hold of a tailor's measuring tape.
I'll poke around to see if there are some sizing guides on-line. If I find any, I'll post it up. For now, I got to get to work!
It doesn't matter if you agree or disagree
... As long as I don't bore you and I spark a moment of thought, my goal is achieved
Queng leon queng tigre ecu tacacut, queca pa? - Pampangan Mandarigma Motto