Most modern cycling helmet comes with an adjustable knob at the back that you can tighten or loosen to fit your head size.
This type of helmet needs to be fitted snuggly/tightly on the head so it doesn’t come off in an accident
Go with the size where the crown of the helmet rest snuggly onto the widest part of your head. Think of it as wearing an actual crown. The crown shouldn’t be too low where it touches your eyebrow nor should it be too high, providing no protection to your head. The helmet shouldn’t tip forwards or backwards. If it does, chances are that when you get into an accident, the helmet will probably slide away from the point of impact. If you fall face first, the momentum will cause the helmet to tip backwards, thus providing zero protection to the front of your head.