Here in the UK the Giro importer is infamous for limited stock
Aerohead in Medium has been out of stock for ages, and not due in until October !
I ordered a Large instead to see what the fuss was about (and see how much the sizes overlap)
It actually feels okay on - but looks huge on my head !

Here is a question; whats the aero disadvantage on sizing up? Is it modest; or enough to return this and choose another model completely?