Wearing a correctly sized hat is important because if not its primary function, which is to protect, would fail. 

First of all, too small or too large a helmet would be really uncomfortable; a helmet that’s too small, in addition to being uncomfortable, would not be able to protect the back of the neck, whereas a helmet that’s too large, in addition to moving uncomfortably on the head, could easily move if you fall off the horse and not cover your head properly at impact. To check if the helmet moves from its correct position when worn, just shake your head slightly with the strap on; if the helmet does not move, then the size is correct.
KEP Italia makes helmet with outer shell available in two sizes, which in turn give rise to all sizes, derived from thickness. Eight different sizes can be inserted inside the average cap, from 51 to 58, 59 to 62 in the large size cap.
Cranial circumference is measured with a tailor’s tape measure, passing above where the ears join the head and in the middle of the forehead.  
But is not enough for a helmet to be the right size, it must also adapt to the shape of the head.
Small spacers can be inserted inside the helmet (in front, behind or on the sides) to adapt it perfectly to the rider's head (which may be oval, round or egg-shaped). KEP Italia made twelve linings for oval heads (typical in Europe) and twelve for narrower heads (typical of North America and Canada). 
It is important to remember to avoid riding with other riders’ hats, unless the size corresponds perfectly to your own. KEP Italia do not advise the purchase of second-hand hats, as their conditions and ability to protect are not known.