Measuring Your Grip Size
The right grip size makes a huge difference in how a racquet performs. If you're not sure of your grip size, here are two simple ways to measure:
Holding an eastern forehand grip (the palm is placed against the same bevel as the string face), you should be able to fit the index finger of your non-hitting hand in the space between your ring finger and palm. If there isn't enough room for your index finger, the grip is too small. If there is space between your finger and palm, the grip is too big. A too-small grip requires more muscle strength to keep the racquet from twisting in your hand. Prolonged use of a grip that's too small can contribute to tennis elbow problems. A grip that's too large inhibits wrist snap on serves, makes changing grips more difficult and also requires more muscle strength. Prolonged use of a grip that's too big can also contribute to tennis elbow problems.
If you don't have a racquet handy, you can also measure your grip size using a ruler. With your hand open and fingers extended close together, align the ruler with the bottom lateral crease of your palm and measure to the tip of your ring finger.
Keep in mind, it's easier to increase handle size on most racquets. In fact, the majority of today's lightweight racquets' handles cannot be reduced in size. If you're between grip sizes, go with the smaller size and add an overgrip to arrive at the ideal fit. A typical overgrip will increase a grip by 1/16 inch. You can also increase grip size using a heat-shrink sleeve. One heat shrink sleeve will increase grip size by 1/8 inch. Both methods will increase overall racquet weight slightly (7-16 grams) but the benefits of a correct grip size far "outweigh" the disadvantages of this added mass.
I also recommend you replace your grip regularly. A fresh, properly wrapped grip provides you with better racquet control and increased confidence.