In a word? Playing with the right grip size is HUGELY important. If the grip is too small and the golfer has to wrap his hands too far around the grip, the muscles of the forearm tighten up. When the forearm muscles get tighter, it becomes much more difficult to take the club away from the ball smoothly and then becomes more difficult to achieve a consistent swing tempo and swing rhythm.
If the grip is too large, this can impede a proper release of the wrist-cock angle on the downswing which also can prevent us from rotating the face back around to square on the downswing.
Over the years, there have been several guidelines created by various golf companies or by people who perform Clubfitting research for measuring the size of the hands and fingers, from which a proper grip size is recommended.
In addition, it has also been taught that the proper grip size is achieved when the golfer closes their hands around the grip and the finger tips come just short of touching the base of the hands. As a starting point, such charts or overlays for the hand are fine. But ultimately each golfer has to decide what their MOST COMFORTABLE grip size should be.
In short, if a hand and finger measurement chart says the grip should be one size and the golfer prefers a different size because it is more comfortable, you go with the decision for comfort – regardless what the golfer’s handicap or playing ability might be. Give you a good example. On average, my wife Mary-Ellen shoots between 95-100. Via her hand and finger measurements, she comes up on the chart for a women’s +1/64 oversize grip. Yet she much prefers and can keep her grip pressure most constant and comfortable with a men’s standard size grip. That’s the grip she plays and that’s the grip I agree as a veteran Clubfitting research person that she should play.
Perhaps the best way for clubmakers to fit grip size for comfort is simply to keep a set of different grip size samples either on cut of shaft pieces or better yet, installed on actual golf clubs. Gripping a cut off shaft section can fall a little short of giving the golfer a real sense of what a particular grip size will feel like when installed on a full length golf club. Most people have enough old clubs laying around that it would be possible to use them as your grip size samples for grip size fitting.
Remember – grip size for COMFORT FIRST so the hands and arms can maintain a secure hold on the grip without excess grip pressure or muscle contraction in the forearms.