The correct way to size a beanie is to stop expanding at : size you want/Pi, for example- If you need a hat with a 12.5in circumference then you would divide 12.5/3.14= 3.98 or 4 inches (point where you stop expanding)
Pinner says: I use this all the time, so just had to repin it for others to learn. Seriously once you master it, youll never want to do it any other way. Also there are lots of photos and a video to help you learn. Foundation single crochet, this is an excellent tutorial for this fabulous method for your first two rows, keeps them from getting too tight, and its very easy..
Learn to Make a Double Crochet Stitch with This Easy Tutorial
If you are *that* beginner like me who goes to crochet tutorials only to find that you need a tutorial for the tutorial, this is a very nice set of pictures that explains AND shows how to do a double stitch.