Most common head size?

[Vanilla]

A mate entered a competition weeks ago and was a runner up. He won a motorbike helmet.

The thing is he doesn't ride, so it'll very likely end up on ebay.

I was wondering what the most common head size is so that he stands the most chance of getting a sale and someone stands the best chance of gettign a deal on ebay

Anyone know the most common size?

Thanks