Tallits designed to be worn around the neck and hanging in front include Size 18, Size 24 and Size 36. This is a reference to the width (i.e. 18 inches wide, 24 inches wide or 36 inches wide), which is a matter of personal preference. The length is standard, 72 inches.
Handmade tallits vary somewhat, but are generally around 24 x 80 inches.
If your bar mitzvah boy is 5' tall or under, you might want to consider a shorter length.