For this simple pattern that is great for beginners ( common and straightforward stitches,) I used Lydia size 10 yarn with a 1.6 mm hook. As long as the pattern is followed correctly you would be just fine using a different sized yarn and hook! My finished piece measured 10.5 inches in diameter, after being blocked which is very important. This pattern is $3 on my Etsy: East End Hookers
https://www.etsy.com/listing/1296102075/lyra-crochet-pattern