Single Crochet a round about 76 stitches or less for your head size. but definately a bit bigger, this is a top hat.
Begin crocheting down Double crochet around (no increasing) for about four rows. Then decrease as many stitches as you will need to fit your head. The top part IS slightly bigger than your head but not too big. Go for as tall as you want.
The brim is tricky. you can use may methods, but my buddy figured make the hat about 4 rows longer than you want and roll when you put on... I however did a front post single and stitched around for a few.... not so great but definately a different look than the rolled look. P.S. mine is all single, my friend did double crochet.