The minimum for 1942 non-waiver was 60". Of post war on terror, it was done with BMI, measuring the circumference of stomach, chest, neck, wrist, and thigh (commonly known as "tape") against body weight, or through body fat density in special situations. I have no clue how it's done today.
U.S. Congress, House. 1944. Committee on Military Affairs. Report of the Commission of Physicians Appointed to Examine the Requirements for Admission to the Army, Navy, and Marine Corps. 78th Congress, 2d session. H.R. 452, Serial Set 10878. February 29. Washington, D.C.: U.S. Government Printing Office.
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