Rome Population:
Ostia, Feeding Rome: Estimates of ancient Rome's population vary widely*, from 500,000 to several million, but however many people there were, they all had to be fed, and most of the food arrived in ships. At first, small boats came up the Tiber to Rome to unload at the huge warehouses (horrea) that lined the river (ruins are still visible.) But soon river traffic became too disorganized, and bigger ships began to unload down at the coast at Ostia -- now Ostia Antica. Ostia was a big and rich city, and now it is a big and easily accessible archeological site. The site is much bigger and has a lot more to see than Pompeii and is also much closer to from Rome. Just head southwest out of Rome on the Via Ostiense (Via del Mare) -- go out Porto S. Paolo (the gate with the Pyramid) and follow the signs to Ostia Antica -- about 40 minutes by car. Or take the 30-minute train ride from Stazione Roma Lido di Ostia just outside the Porto S. Paolo. Trains leave every half hour -- get off at the Ostia Antica stop. If you want to know what you will see before you get there, browse around Ostia Antica's great Internet site at http://www.ostia-antica.org/, one of the best archeological sites on the web.

 
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* Ancient Rome's Peak Population:  There is much scholarly vituperation about how many folks really lived in Rome, but the guesstimate that I like best is based on food distribution in the time of Diocletian.  I like it because it gets past the questions of how much of the metropolitan area should be included in "Rome" (the walled city? or the official administrative "neighborhoods"? or the urban sprawl?) and whether census figures included everybody or just free male heads of Roman households.  It's important to know that Diocletian was a great fiscal reformer and something of a nut about keeping accurate records.  It is also important to remember that contemporary accounts say that Ancient Rome reached its peak population about that time.

The math is complex, but this is the way it works:
 

first, almost nobody worked, so almost everyone in town was eligible for and took advantage of the food dole;

second, there are records of how many containers of food were distributed in the neighborhood distribution centers;

third, the size and capacity of those food containers was strictly regulated and enforced;

fourth, there are historical accounts of what and how much people of the different classes ate;

fifth, there are also accounts of how the population was divided (family make-up in the several citizen classes and in the freedmen and slave classes);

So, simplified,  you take the amount of food distributed and divide that total by the amount that each person ate, and you get an approximate number of inhabitants.  Then you do another calculation, starting with the records of how much food was passed through the horrea that lined the Tiber, to check your accuracy.  Both sets of computations give about the same number: 1.5 million souls.

In case you were wondering, the current population of Rome is about 3 million, but there are still the same disputes about how much of the urban sprawl to include in the definition of Rome.