Getting a properly sized motorcycle helmet is more involved than guessing “Small," “Medium," or “Large."
In general, a brand new helmet should feel slightly tight, coming into contact with most of your head and the sides of your face, but not putting too much pressure on any specific area.
Once you've worn it for a while, your helmet should be “broken in" and adjust to the specific shape of your head, but it should never become loose.
To find which size of helmet to start with, grab a tape measure and get the measurements for the largest circumference of your head, which should be just above your eyebrows and above your ears.
Consult a sizing chart (you can find several online or through your motorcycle equipment retailer) to see which size helmets you should be trying on.