A bicycle helmet only offers sufficient protection if it fits properly. So choose the size best suited to your head and the most comfortable.
To do this, the ideal is to measure your head circumference at the level of the forehead (where the diameter is the largest), approximately 2.5 cm above your eyebrows and your ears, using a flexible tape measure, as shown below. If you don't have a flexible tape measure, use a string or shoelace that you can then measure.