You need to wet it, then get it to dry slowly over a form of some sort so it'll dry to the required size; I've used footballs, dinner plates, all sorts of things, but the best has been an upside down large mixing bowl with your head size markad around the outside. Pull the wet hat down over the bowl until it's come to the right place, then let it dry slowly and go often to ensure it doesn't slip back up to a smaller diameter.
This method has worked well for me in the past, hope it helps