Motorcycle helmets come in all shapes and sizes. The type of motorcycle helmet partially determines the fit and comfort. Each type of helmet has its own specialty. Think of a full-face helmet for racing, a motocross helmet for off road and enduro rides and modular helmets for convenience while touring.
To determine the right size for a motorcycle helmet, you need to measure the circumference of your head. This is best done above your eyebrows. The measured circumference in centimeters gives you an indication of what the correct size for a motorcycle helmet would be. I can require some skill to measure it correctly, so please make sure you measure it a few times to get a real good indication. You can find the sizes which brand use in cm below. Are you in between sizes? Then go for the smallest one.