Grip fitting, all depends on anatomy and type of swing. ONLY place where you are in contact with the full swing clubs. There is no one type of sizing that fits ALL. Can't do it to say do it my way and the rest of you are all wrong. ALL YMMV.
I have found some generalities with single planers ala Bryson with grip more in palm larger grip size more upright plane (depending on other stats.) grip size helps keeping single planers on plane better. Just an observation. Double planers all over the place.