I'm really excited about the Civil War Provost Marshall Records at the National Archives. While at a conference recently, Jake Ersland of the Central Plains Branch of the National Archives, gave a talk on these records (Record Group 110). What a wealth of information they contain!
In 1863, the Provost Marshall General Bureau was established and they were to produce lists of men eligible for the draft, examinations of eligible men, descriptions of the men, arrests of deserters and rejections.
The states were divided into districts and then into sub-districts which were responsible for visiting all the males and determining their status and processing those who were enlisted or drafted. This bureau produced the following lists:
Consolidated list of men eligible (subject) to draft - this gives you a broad look at all the male
individuals within a district.
Register of Drafted Men, Recruits and Substitutes
Medical Registers of Examinations of Recruits and Substitutes - a description of health and features
Register of Medical Examinations Showing Rejections and Exemptions
Descriptive Book of Deserters
Veteran's Reserve Corp.
So, if you know where your Civil War era ancestor lived, accessing this record group could get you a wealth of information on their description, why you can't find them on any Civil War unit rosters, and other family member living in the area.
Unfortunately, these records have not been microfilmed. To look at them requires on-site research at the appropriate National Archives branch. If you can be specific enough, you can contact the National Archives holding the records for the region your ancestor lived and researchers at the archives will copy the information for you. If you are unfamiliar with which National Archives holds the records for the area of the country you are interested in, you can find the information on the archives.gov website.