Though this guide will mostly focus on nonclothing items for 5-year-old boys, if you are interested in purchasing clothes, a good rule of thumb is that 5-year-olds boys wear a size or two within their age. But of course the best way is to go by measurements: • size 4 or S42 44 inches; 37 – 41 pounds • 5 or M45 – 47 inches; 42 – 46 pounds • 6 or L48 – 49 inches; 47 – 53 pounds