Working on a crochet hat? Here's a great link with a sizing guide for any hats (or headbands) you may have on your hooks. This includes sizing from babies to adults!
While this doesn't have a certain number of stitches in a round for each size, it's hard to do that because everyone crochets with a different tension. I'm willing to bet you hate finishing a project only to find out it's too small or way too big. As you are working your hats, measure the size (circumference) after each round. This will ensure that your hat will fit properly.