Most helmets are so adjustable, it's almost impossible to buy he wrong size -- now shoes are a different question!

My Bell Slant was "universal fit" -- basically one size fits most, and it does a good job.

Now that I know what to look for in shoes, it's easier to pick online, since the online 'shops' have comparison charts for different brands. Just do the comp between the brand you know vs. the brand you're looking at, and your chance of success goes WAY up.