A crochet hook size provided in the Material’s List of your pattern is very important information.

HOOK SIZE + YARN SIZE will determine the projects FINISHED SIZE (approximately, your own tension will alter size too). Doing a practice swatch before beginning and measuring periodically as you crochet your project, will keep you on track. Look for GAUGE information in the pattern.

Because we want lace to be FINE and/or  sweaters to be BULKY you need materials and hooks in different sizes to achieve the desired and expected result.

Hooks come in steel, aluminum, plastic and bamboo. You should try what feels good in your hand, if you find your hand is getting tired or stressed it could be the wrong hook (or you’ve been crocheting too long!), switch to maybe a bamboo that will be warmer in your hand.

Be cautious when choosing and switching to stay with the same manufacturer – they all do size a bit differently.

Check out this link regarding hooks and needles.