If in doubt, or if you feel like you fall in between two grip sizes, we recommend to choose the smaller grip size because you can easily build up a grip if necessary. Also a smaller grip leaves room for the use of an overgrip.
In general, smaller grips allow more freedom of movement and quicker change of grip (forehand / backhand), and they facilitate effective wrist use – especially for technically demanding strokes.
On the other hand, a larger grip requires less muscle strength to prevent the racquet from twisting in your hand – especially if you hit the ball off-center. That´s why prolonged use of a grip that's too small can contribute to „tennis elbow“ problems.