there are many head shapes and sizes in this big 'ol world ... what is your size?  here is a basic way to measure your noggin and find the size that fits you. 
Step #1: Get a tape measure
Step#2: Measure around the largest part of your head ~ with the tape measure about 3/4" to 1"  above the top of your ear.
Step #3: Take that measurement and compare it to our size chart below and find your size.