The "One Size Fits All" caps will fit everyone. They have a strap in the back of the cap that allows you to adjust the cap to the proper size.
To find your perfect fit, take a measuring tape and measure around your head where the cap will sit, usually just above your eyebrows, consult the table below to find the right cap size.
If more than one size is possible, select the one that best suits your tastes. Remember you can always return it if it doesn't fit you and try another size.