Smith Variant Helmet - Adult 2010
This is the helmet that I use. Check the video that we have on there. It teaches you how to properly measure and fit a helmet.
If the Variant is out of your price range, look into the Smith Maze. Both helmets are very light, with the Maze being the lightest. Smith makes high quality helmets and I can tell you first hand that it protected me from a very bad fall. I am 99.9% positive that I would have been hospitalized if I didn't have it on a few weeks ago.