Tennis racquets have different grip sizes and I am not sure which one would be the best for me. Is there an easy way to find this out?
There are two rules of thumb to determine your correct tennis racket grip size:
Hold the racket by the handle as you would while playing and see if you can insert your your non-dominant index finger in the space between your palm and ring finger tip. If your index finger is snug, the grip is sized correctly.
A player can measure his or her grip size with a ruler. Measure the distance from the tip of your ringer, down to the horizontal crease in your palm, closest to your wrist.
These methods are described in greater detail with photographs here: http://www.tennis-warehouse.com/LC/Gripsize.html
There is a trend right now among tennis players to actually play with a handle that's one size (1/8th) smaller than what size you would arrive at using the methods described in the answer provided by Dirty-flow. This allows for more a little bit easier wrist snap/movement. For example, based on my hand size I should play with a 4 3/8 size grip but I actually find it more comfortable to play with 4 1/4.
The other thing to think about is how much of the bevels of the grip you like to feel in your hand, or if you like to feel a more smooth, round grip shape in your hand. Different racquet manufacturers have slightly more or less oval shape to their grips than others. Head racquet grips are more oval while Wilson grips are more octagonal.
At the end of the day, you just have to feel comfortable with the overall feel of the grip and that it shouldn't move too much (if at all) in your hand. You'll need to try this out for both the forehand, backhand and serving grip (preferably by hitting a tennis ball) because you can also feel the grip against the impact of the ball.