Grip size is one of the most important fitting factors. The grip below is the proper size, note the left ring finger just touching the pad of the left hand. (For the left hander it would be the right hand.) The second photo shows the grip is too small and in the bottom photo the grip is too large. If the grip is too small you will pull the ball and hook. If the grip is too large you will push the ball and slice.

This is the proper grip size

 This grip is too small

This grip is too large