Size of the U.S. Army, 1914-1920
This includes all military personnel on extended active duty with the Army (regulars, volunteers, militia, inductees, reserves and national guardsmen) and U.S. Military Academy cadets. This does not show the actual strength of the American Expeditionary Force (AEF) in France nor does it reflect the total number of men (4,734,991 mobilized) who served at some time in the military (army, navy, marines) during the war.
The line graph shows the incredibly fast expansion of the army once war was declared and the equally swift demobilization after the armistice of 11 November 1918. Once the U.S. Senate rejected ratification of the Treaty of Versailles on 19 November 1919, there really was no need for an expanded army as the U.S. shrank from playing a more active role in world affairs.
Sources: U.S. Department of Defense, OASD (Comptroller), Directorate for Information Operations and Control, Selected Manpower Statistics, (May 1975), page 23 (https://archive.org/details/DTIC_ADA043053/page/n25)
Size of the American Expeditionary Force (AEF) in France
Note the extremely rapid build-up of American soldiers in France through 1918. That was a very impressive ramp-up of operations by the United States.
Sources: Leonard P. Ayres, The War with Germany: A Statistical Summary (1919). See also the Annual Report of the Secretary of War, 1919, or Michel Huber, La population de la France pendant la guerre (1931), pages 121-22.