Racing helmets are a personal item that needs to feel comfortable on your head, so don't choose one just because it looks kickass - Though admittedly that's what I did with my first helmet... I got lucky it fit perfectly and was comfortable.
No matter what you race be it a Mazda Miata, Corvette, Sprint Car, INEX Legend or a Prototype fit is the most important thing.
HERE'S HOW TO MEASURE YOUR HEAD:
a. Use a cloth tape measure or a string to make your initial measurement.
b. The circumference of the head should be measured at a point approximately one inch above the eyebrows in front and at a point in the back of the head that results in the largest possible measurement. Take several measurements to make sure you have the correct measurement.
c. If you have a beard or an unusually thick head of hair, you may want to consider ordering one size up from what is shown on the sizing charts, especially if you are near the top of your size range.