There are two ways to determine what grip circumference you need, the Ruler Test and the Index Finger Test.
To measure your grip size using the ruler test, first place the fingers of your paddle hand together, then align a ruler’s edge with the bottom horizontal crease of your palm. Next, measure to the tip of your ring finger. This measurement is your grip size.
Index Finger Test
To measure your grip size using the index finger test, hold your paddle in your hitting hand. Place the index finger of your non-hitting hand in the space between your ring finger and palm. If there’s not enough room to fit your finger in this space, the grip is too small. Conversely, if there’s a large gap between your index finger and the ring finger and palm of your hitting hand, the grip is too big.
Ultimately, you want the index finger to fit snugly between the ring finger and palm of your hitting hand, with little or no space between. This will give you the most comfortable and secure grip.