The best source to answer this question is "The War with Germany: A Statistical Summery" by Colonel Leonard Ayres (1919) The bullet points are this. The American Expeditionary Force deployed slightly more than two million men to France, with most arriving in the last five month of the war. 1,390,000 soldiers were in the 42 divisions that actually made to the front and this includes 214,000 soldiers that were in the rear echelons that were close to the front. These rear area troops were in the Army air service, or they had staff jobs at the various headquarters.
The US Army in 1918 was all infantry divisions. They were oversized by the standards of the British, French and German divisions in 1918. Each division had four regiments and each regiment had four battalions. These 16 battalions had a lot of support personal, primarily artillery. 28,000 troops were the compliment of a brand new American division, when it first went into battle.