I have done a few, found most of the techniques on internet. For sizing, average the front - back with side - side measurements of your head then add 3/4". Make the outside diameter of the band area this measurement. That number is based on the hat band being 1/8" thick and the shrinkage taking up 1/2". The rest of hat should be less than 1/8" thick (3/32" is about right).
You will need to enhance the oval some (or a lot depending on head shape) with a jig or clamps while it is drying. It is a fun project but expect a few failures along the way.