In THE BODY SIZE OF SOLDIERS _ U.S. Army Anthropometry-1966 is some craniometric data from 1946 (and 1966):
Meanwhile in 2012 ANTHROPOMETRIC SURVEY OF U.S. ARMY PERSONNEL: METHODS AND SUMMARY STATISTICS we get comparable stats from 2012:
By 1946 standards,adult male U.S. head circumference, head length, head breadth and head height increased by 0.5, 0.64, 0.39 and 0 standard deviations (SDs) respectively.
Using Lee & Pearson’s formula for guessing cranial capacity from external head size measurements, volume increased from 1456 cc to 1499 cc, an increase of about 0.47 SD in 66 years.
However the true increase is likely larger, because in 1946 there was a draft so the upper class (who have bigger heads) were represented in the army. In 2012, not so much.
Also, changing U.S. demographics, particularly immigration from poorer countries where people are smaller, would negate some of the increase in the non-immigrant population.