Just like with any other hat or beanie, there are plenty of ways to make sure you buy the right size for your hat. You can compare the size written on an older hat or cap that suits you well with the size of the hat you want to acquire.
You can also measure the circumference of your head and compare it with the sizing chart provided by the manufacturer. If the hat you want to acquire has a universal size, you can adjust it according to your needs from the hook and loop closure or the velcro closure on the back.