Fitted baseball caps do not adjust to different head sizes. This cap type should be purchased in the right size for you. If the cap is made from cotton or wool, that means you should buy a cap slightly larger than your head size. These fabrics shrink under the heat and moisture produced naturally by the body, resulting in a smaller end-product that eventually conforms to the unique contours of your head, producing a snug fit. Most baseball caps produced today, however, are constructed from polyester, removing the need to allow room for shrinkage. So buy a cap in this material to fit your head exactly and wear your cap as often as possible to break it in quickly and enjoy the fit.