Hi. Your hat is too big, huh? Sounds like a gauge problem. When you can, please post a link to your pattern, you'll be able to after a few posts. Your pattern should give a suggested hook size and a yarn type, along with how many stitches per inch (gauge) you'll need to get for the hat to come out the right size. You can use a smaller hook to get more stitches per inch. It sounds like your hook and/or yarn are the wrong size.
Bev's Country Cottage has size charts. This is from it. You would fold the hat and measure across. Multiply it by two to get the total head circumference. The length of the hat includes a one inch cuff that is folded up. Measure the width as you crochet and stop increasing when you reach the proper circumference.
Child (3 - 10 years)
Head Circumference: 19" - 20 1/2" 48-51 cm; Hat height =8.5" 22cm
Just starting with fewer chains or stop increasing sooner is great if this is a stocking cap type of hat, but others are not constructed in the same way. That's why I was hoping for a pattern link. I hope the OP comes back and lets us know how it's going.