It is very important to use a racquet with the correct grip size. Unfortuantly most people do not know how to select the correct grip size for their racquets.
The grip sizes for adult racquets start at 4" and go up to 4 3/4".
4
4 1/8"
4 1/4"
4 3/8"
4 1/2"
4 5/8"
4 3/4"
You may not understand the sizes scale in some countries as they use numbers:
#0 = 4"
#1 = 4 1/8"
#2 = 4 1/4"
#3 = 4 3/8"
#4 = 4 1/2"
#5 = 4 5/8"
#6 = 4 3/4"
The Junior racquets sizes are from 3 5/8" to 4" and in some cases to 4.1/8.
The best way I have found to determine your grip size is to print the grip sizer from Wilson's website:
http://www.wilsonsports.com/media/wilson/pdf/gripsize_9861.pdf