NOT one size fits "most", it is a size "small"
After using this cap for a while, I kept thinking I was putting it on wrong.  But, I finally checked inside the cap and found a little tag with "S" on it.  No wonder it kept slipping off and loosening the strap so that my jaw would not stay closed!  I have found that other sellers offer two sizes, and I would have ordered the large size if it had been offered on this item listing, but it isn't.  I will attempt to return it.<br />UPDATE<br />I changed my review from 1 star to 4 stars because after contacting the company, they exchanged the small size for a Large size at no cost to me, so excellent customer service.  However, other vendors which sell both sizes include instructions in their description to measure a couple of head dimensions to determine the correct size to order. The large size works great for me and does not slip off my head since the rear Velcro straps now correctly fit the shape of my head.  This design of chin strap does not put concentrated pressure on any one part of your head, which means that your hair is not smashed down in one location, which can damage the hair follicles.  The cap also does not make my head feel hot, I guess because of it's lightweight cotton material.  All in all, when sized properly, this is a good product.
Rating: 4 stars