Rob:
Shape of the helmet might affect fit as much or even more than the actual size.
Arai makes three different shapes of helmets (meaning, three different interior shapes, to cater to different head shapes). You might want to check out their web page.
Also, on the higher end Arai's, you can remove the ear pieces and the cap over the top of the skull, and substitute these for other sizes, larger and smaller.
This isn't exactly made to measure, but it does offer a heck of a lot more choice than most manufacturers.
I own 3 Arai's, and I am very happy with the fit and quality of them.
Michael
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