You can use any yarn weight or type, with a suitable hook size to make your mini hat. As a guide, I have used Paintbox Yarns Simply Chunky (US Bulky), 100% acrylic yarn, with a 6mm crochet hook. My finished hat measures approximately 13cm/5″ wide across the brim, 8cm/3″ tall, and has a inner head diameter of 6cm/2.25″.