All guitars (acoustic, classical, electric) come in different sizes: full (4/4), 3/4, 1/2, 1/4.  Full size is the most common size and has the most choices to choose from.  3/4, 1/2, and 1/4 sizes are for kids.  All adults, regardless of their size, use full size guitars.

Choosing the right size guitar to use depends the arm's reach of your child.  You can use age to determine the approximate guitar size for your child using the following chart.