I use a #8 American needle with a cast on of about 90 in worsted weight. I like to do mine with a rib stitch, either k1, p1 or k2, p2, whatever you prefer. This hugs the head and stays on better. The experts recommend you figure out your gauge per inch and multiply that for your head circumference. Subtract one inch so it fits better. Knitting will stretch. If you do it exactly according to your head size, your hat will twist around and ride up. You'll constantly be pulling at it to readjust it. It's annoying.
Here's a great one size fits all hat pattern that worked out very well for gifts this past Christmas. It hugs your head like it was custom made. A hint: Use a German twisted cast on for cuffs for hats and mittens. A regular cast on won't stretch enough to allow you to get it over your head or on your hands. Amy has an excellent tutorial for it at the top of the KH site right here under the Casting On section.