I found it best to go to a motorcycle accessories shop and try them on, they all fit slightly different and some were uncomfortable compared to others. I noticed the strap makes a big difference, depending on where it sits under your chin - some had bigger clips that seemed to dig into my throat too much!
Got mine from
http://www.ashtonsaccessories.co.uk/Motorcycle-Helmets. Know thats of no use to Scotty
but would recommend. Bought a Caberg full face that's done me fine for 6 or 7 years of track stuff and was only £95 i think. Just remove the visor when in a car as its more comfortable with breathing etc.