Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Go to a shop and get one fitted.
Needs to be snug enough that is doesn't wobble around if you hold it and shake your head.
Then buy online.

I bet local bike shops love you!

Im as tight a person as your likely to find, I barter on everything and never pay asking price.

Yet for the £2 or so discount youd get from ordering it online, Id rather just hand it over to the shop which has just put up with me trying on all their helmets.