Hats have adjustable snapback and one size fits most.
Leather is treated with a wax coating to make it as weatherproof as possible, however this is genuine leather so treat it accordingly. Over time the leather will change with exposure to the elements as well as potentially sweaty heads… (men, we’re looking at you). But no special treatment is required – as long as you love it, wear the heck out of it! You can always buy another one 😉