One way is to decide on your cities and not worry about reality. Another is to determine how much and what quality land you have and then figure out how big a city it will support. Another is to read up on some rules of thumb and use those.

http://www.history.ac.uk/resources/e...rs/keene-paper seems to be a fairly nice paper on basic human development in the medieval period.

A single large monster like a dragon can probably be considered equal to a small city in terms of its food requirements. Simiarly, races without an agricultural tradition (goblins and orcs being the traditional examples) will tend to raid agricultural fields, putting additional strain on resources.

On the other hand, it may be quite likely that fantasy crops are higher-yielding and more disease-resistant than their medieval equivalents. Very high quality forage plants, for example, might well allow raising animals on relatively small plots of land. Grains that produce their own fertilizer (e.g. a form of nitrogen-fixing wheat) would reduce the need for allowing fields to lie fallow 1/3 of the time. Meat trees (the fruit is primarily protein dense enough to be equivalent to meat) allow even the most vegetarian race to grow fast enough to keep up with the carnivores. And so on.