Does anyone have any sources mentioning the percentages of citizens at various times, or the populations of citizens and corresponding total populations so I can calculate percents? I looked at dozens of sources online without finding this kind of information.
I found two estimations about the number of slaves for the 2nd and 1st centuries BC.
- P. Brunt  estimates there were 3 000 000 slaves in a total population of 7 500 000 people.
- J.C. Dumont  estimates that at least 32% of the population was in slavery.
From these numbers, we deduce that roughly 60% of the population were free. Halve this proportion to exclude women, further exclude children, and the proportion drops to 20-25% of free men (I have no precise idea for the proportion of children). Not all of them were citizens, but at least we have an upper bound.
: Italian Manpower, 225 BC -- AD 14, Oxford, 1971
: Servus. Rome et l'esclavage sous la République, Rome - Paris, 1987