My hats are wearable artworks rather than designed for hard work. I spray them with a product that makes them rain and dirt repellent but NOT rain and dirt proof. If you spill something on your hat, gently sponge it off as soon as possible. Do not wash or dry clean your hat. You can treat it every few months with a protective spray or let it gain a patina of age.
There are a number of websites that explain or show how to measure your head for buying a hat if you can't try it on first. I find this one helpful.
I prefer decorating wool felt hats because the fabrics adhere well to the fibre however I have worked on two cotton hats on commission (appliqueing the rim only because the crowns were too soft). I am testing the adherence of fabrics to other types of hat.
I rescue all my hats from charity shops, so I never know what shapes and sizes I am going to bring home. This is a wonderful way to upcycle hats that might otherwise end up in landfill but it does mean that I cannot offer hats order.