In the US Army during WWII, logs were kept by the Quartermaster Corps of what sizes of clothes were issued for supply purposes, as well as by the Medical Corps for statistical purposes. It was found that the average male inductee was 5 feet, 8 inches tall and weighed 144 pounds; an inch taller and eight pounds heavier than his Great War counterpart. He had a 33 1/4" chest measurement and 31" waist measurement; he could expect to add an inch to his chest and gain anywhere from five to twenty pounds during training. Many inductees were underweight due to insufficient nutrition as they grew up during the Great Depression.

From the tariff tables showing the frequency of size issues it is found that the sizes most frequently issued are a 7 to 7 1/2 hat, number 9 gloves, a 15 shirt with a 33" sleeve, a 36 regular jacket, a pair of trousers with a 32" waist and a 32" leg length, size 11 socks, and size 9-D shoes.

Sources:

G.I.: The American Soldier in World War II, by Lee B. Kennett

Tailor to Millions, by Harold P. Godwin (Quartermaster Review, May-June 1945)