Size is determined by type yarn and size hook used. For small size (about 3.5 inches in diameter) use about 1/2 ounce fingering yarn and a size D crochet hook; medium size (about 4.5 inches in diameter) takes about 1 ounce sport weight yarn and size E hook. For large, (5.5 inches in diameter) about 2 ounces worsted and size H hook.