An integral component of motorcycle helmet retention is the fit. Most motorcycle helmet manufacturers will provide advice for fit and sizing along with the helmet itself.
Dealers, in addition, are often trained in the art of helmet fit. A reputable motorcycle helmet dealer will inform you that a helmet should fit tightly so as not to shift if you were to shake your head about.
A full face helmet should contour to your cheeks and jaw as well as the top and sides of your head. Too loose of a helmet is an accident risk and too tight of a helmet may be highly uncomfortable. A good test for proper helmet fit is that a snug helmet moves the skin on your face as you try to move it on your head.