Ancient Rome: A Military And Political History, covers it iirc, speculating that the voting assemblies and divisions mirrored (they actually were), the military units. So by the Comitia Centuriata, there would have been 100 men units...
Unfortunately, exact details for those years are scarce (if there are any lol) and one must remember the roman practice of writing contemporary institutions, into the past, or "fixing" them, so that they would work in or explain the present ones...