Manufacturer's Sizing Advice

Here are links to information about fit.  A couple sites list a size chart and fit advice while others have just fit advice. 

Klim (shown on each product page)

Nolan (no sizing advice on Nolan's site)

Scorpion

Shoei (shown on each product page)

Speed and Strength

AGV

Arai (no sizing advice on Arai's website)

Bell Helmets

HJC

 Icon  (shown on each product page)