This is only a rough guide to sizes - it changes depending on the manufacturer and shirt in question.
KIDS SHIRTS
Small - chest size 26-28″ (Typically age 7-8)
Medium - chest size 28-30″ (Typically age 9-10)
Large - chest size 30-32″ (Typically age 11-12)
XL - chest size 32-34″ (Typically age 13-14)
ADULT SHIRTS
Small - chest size 34-36″
Medium - chest size 34-40″
Large - chest size 42-44″
XL - chest size 46-48″
XXL - chest size 50-52″