Manufactured tennis racquet grip sizes range from less than 4" for juniors to 4 7/8" for the largest adult hands. This might not seem like much of a range, but the difference even 1/8" makes is surprising. Prolonged use of too large or especially too small a grip can injure your hand, wrist, and elbow.
The most commonly used method for finding your exact grip size is as follows: On your dominant hand, note that your palm has three main creases. Hold your hand flat, with the fingers alongside one another. Measure from the middle crease of your palm, up the line between your middle and ring fingers, to a point equal to the height of the tip of your ring finger. For most women, this measurement will fall between 4 1/8" and 4 3/8", for most men between 4 3/8" and 4 5/8". Juniors will usually measure less than 4".
Generally, if you're between eighths when you measure and you're not growing, you'll be better off going with the smaller grip, as a slightly small grip can be fattened up by 1/16" easily with an overwrap, whereas a too-large grip would have to be shaved down at a pro shop, and some racquets can't be shaved down. Overwraps can't fatten a grip effectively more than 1/8" though, because each layer of overwrap adds to the rounding off of the bevel edges on your handle.
Grip sizes between 4 1/8" and 4 5/8" are easy to find in adult racquets. Larger and smaller grips are made for some racquets, but not all. Players with unusually small or large hands often have their racquet handles custom-sized at a pro shop, which should cost between $5 and $15. You can also get a do-it-yourself grip enlargement kit.
For junior racquets, exact size matching can be difficult. Most junior racquets are quite inexpensive, and the manufacturers don't find it economical to produce a wide range of grip sizes. Often, the best you can do is to get a racquet that's fairly close. For juniors, grips slightly large are preferable to slightly small, because they will grow into them.