You can choose any yarn you like to crochet clover square motif. Just to give you an idea of the size, my purple square is crocheted with 6 strand embroidery floss and a 2,5 mm crochet hook and it measures about 10 cm (about 4 inches).
Besides your thread or yarn and the appropriate hook, you will also need a needle to weave in loose ends and a pair of scissors.