Getting the right lid for your skull isn't tough, you just need to measure correctly. It is important that you get an accurate measurement, it is possible to make reading mistakes of up to 2cm, so be sure to measure the biggest circumference of your head accurately.
The following should insure that you get the right fit: Take a soft flexible tape measuring roll (or print out one below) and measure the circumference of the forehead an inch or two above the eyebrows (or wherever is the biggest.) Be sure to take the reading OVER your hair, unless you are going to cut it all off before you wear the helmet.
Have a friend read the distance or grip the location with your fingers and read the number yourself (be sure to read to at least 1/8" or 1/4cm.) Repeat 2 or 3 times to be sure you have measured correctly. Convert from inches to centimeters by multiplying by 2.54 (that is 24 inches = 24X2.54 cm.)