Re: Historical Census
Many population estimates, espeically for the Classical and Hellenistic Greek cities, such as Olynthos, Athens, and Piraeus, are done by looking at housing and city layouts. There are also extensive records for many cities of their citizen population for voting purposes.
If you have a citizen population number, you can probably get a good ball park of the full population including the women and children of the citizens, their slaves, and any metics.
For example the Spartan citizen population at its height was at around 13,000, but add on their families and their massive slave population and it was much larger.
Athens, which was much more liberal (by Spartan standards anyway) about their citizenry had about 60,000 at its height in the mid 4th century BC. The population of greater Athens in its entirety based on multiple factors (we know usually how many slaves people had, birth rate so we have average family sizes etc) was probably around 500,000. About 150,000-200,000 inside the city limits of Athens, the rest in towns and villages in Attica.
There were about 40,000 Athenian citizens during the 5th century BC.
By looking at all the individial city states in Greece, Sicily, and Asia Minor, historians generally estimate the Greek population in the 4th century at about 10million... about 8million a century earlier. (1million in Sicily, 2-3million in Asia Minor, 6-7million in Greece). Compare that number to the population estimates of Greece in 1000AD at 500,000... for the whole of Greece.
Morgens Hansen has done a lot of great work on Greek demography. One good work on it is his The Shotgun Method: The Demography of the Ancient Greek City-State Culture from 2006.
I'm not as familiar with Rome, but I think the historian Durand (I think?) did a study that put the population of the Empire at the end of Augustus's reign at about 60million... but Rome is a lot harder to estimate than the Greek Cities that's for sure.
The historian Walter Scheidel has done a lot of work on Roman demography as well, I can't think of any titles at the moment but he has several published works.