For an adult female, the hat should fit if you crochet a circle until it measures 5 FLAT (stop your increases at that point). For a man's hat, stop you increases when your circle measures about 6 FLAT.
Great reference for hat sizes! - Hat sizing chart Age (Diameter of circle) Newborn in) months in) months in) Toddler/Sm Child Adult Woman Age (Finished Length) Newborn months Toddler/Sm Child Adult Large Adult
Pattern a lady used for Chemo hats and she used I love this yarn. Chain up 2 for hdc. Join with slip stitch and ch 2 but do not count it, will go over it and will be hidden but will make a straight seam.