To find which grip size you need, you can take a racquet in your hand and check that it remains the size of your index finger between your hand and the tip of your ring finger.

If there's no space, the grip is too small.

If a large space remains and you can move your index finger in that space, the grip might be too big.

A grip that is too small or too big is a problem to develop a correct technique and it may induce injuries, forcing you to hold the racquet too tight.

The grip size is also a parameter that is somehow personal ; some tennis players will prefer a grip a bit smaller and some others will prefer a grip a bit larger.

Tennis players with a flat game tend to prefer larger grips, while players who use more top spin tend to prefer smaller grips, that facilitate the movements of the forearm and the use of semi-western or western grips.

General tips concerning the grip size

Most of the time :

• For kids (younger than 11 years old), the grip size is always 0 (4 inches),
• for women, the size if often 0,1 or 2,f
• for teenageers, the size is in general 2 or 3,
• for men, players often use the size 3, some will also use the size 4 or 2, more rarely 5, depending of the size of the hand, the game style and the personal preferences, 
• the average grip size for men profesionnal tennis players is 3, 2 for the women.

In case of a doubt, I advise you to choose the smallest size, because it'll be possible to increase a bit the grip size, and it won't be possible to decrease the grip size.

If you plan to use an overgrip, take that into account because adding an overgrip generally increases the grip size by an half-size.

In Europe, tennis grips sizes vary from 0 to 6.

In the USA, the grip sizes are expressed in inches.

This is a correlation table with the grips sizes in Europe and USA.

Tennis grip sizes can differ a bit between tennis brands so it's always a good idea to check in real if a grip sizes fits your hand.