The Centuriate assembly (Comicia centuriata) was arguably the most important assembly of the Roman republic. It was in charge of electing consuls, praetors and censors, of voting laws, declaring war and peace, judging capital cases, etc.
The Centuriate assembly included all Roman citizens, but not all votes carried the same weight. Citizens were organized by centuries according to their role (if any) in the army, and centuries had different sizes but each the same voting power. There were 193 centuries (at least during most of the republic), 18 of equites, the richest citizens able to buy a horse for war, 170 of infantry divided in 5 classes according to the weapons they soldiers could buy themselves, 4 of diverse people, and one of proletarii, too poor to be in the army.
All this information is in Livy (perhaps not as detailed there), on wikipedia, and almost in every modern book on the Roman republic I looked at. Obviously, the centuries of equites were composed of fewer citizens that the centuries of first class infantry, which gave each of the member of an equites century a greater individual voting power, etc...
But a question that I have not seen answered is :
How many citizens was there in each century of the different types?
Of course there is no definite answer because it must have change during the long life of the roman republic. And it may be difficult to know precisely. But has anyone made some serious estimates on the question, say for certain dates like 400BC, 300Bc, 200BC? The question seems important in order to evaluate how biased toward the richest citizens was the centuriate assembly, and the Roman republic as a whole (which of course had other biased institutions, like the senate).