I suppose one question that I've run into. They list the helmets in the size:

• Small (S): 51-55 cm / 20-21.6 in
• Medium to Large (M-L): 54-61 cm / 21.3-24 in
• Extra Large (XL): 61-65 cm / 24-25.6 in

I've tried to measure my head, and my measurements vary between 23 1/2" and 24".

That puts me right on the division between ML & XL.

I'm leaning towards XL and using slightly thicker pads if I can. Let's see, circumference = πD. So, an extra inch in circumference equates to a little over 1/4" in total space around the head, distributed to half of each side of the head.

I suppose I should ask them if the shell sizes are the same and the foam is different between models.