Crochet hooks are sized for different weights of yarn and thread. For thread crochet, you will usually use a steel crochet hook. Steel crochet hook sizes range from size 00 to 14. The higher the number of hook, the smaller your stitches will be. For example, a size 1 steel crochet hook will give you much larger stitches than a size 9 steel crochet hook.
Keep in mind that the sizes given with the pattern instructions were obtained by working with the size thread or yarn and hook given in the materials list. If you work with a smaller hook, depending on your gauge, your project size will be smaller; if you work with a larger hook, your finished project's size will be larger.