Helmets are adjustable for all sizes. The padding in them can be changed to smaller or less padding to accommodate different head sizes and shapes. Look for a helmet padding kit and youll be able to fit it to your head

 

Yes, as protectionperfection said, most helmets should fit most people. This way they don’t have to make different sizes. Have you tried on a helmet? Was it too small?

 

Try a PASGT. The harness system isn't as secure as padded helmets, But im' pretty sure it will fit any size head...except for Stewie Griffin's.

 

Most replica helmets are going to be medium to large size. Even changing the padding isn't going to help your size 8 head (I know - I'm a 7 3/4.) The closest I got was the Ops Core base jump and even then I had to trim some of the foam from the front of the helmet. The other helmet I have is a Gentex SOHAH and that one will definitely fit you but it is RS and obviously much higher price. Mostly you will have to look at real XL or XXL helmets on Ebay.

http://www.gentexcorp.com/default.aspx?pageid=2710

http://www.ownthenight.com/catalog/i92.html

Remember that a helmet in airsoft isn't going to be used for ballistic purposes but it still needs to fit correctly 1) for the look and 2) to provide bump or fall protection.