The territories ruled by the Norman kings covered an area of about 100,000 square kilometres, slightly more than Scotland and Wales combined, or about three quarters of the size of England. Estimates as to the size of the population depend on some imperfect information about late thirteenth-century taxation which yielded an unexpectedly high figure of about four and a half million. Given the proportion of mountain and pasture-land in the South, some districts must therefore have supported impressive numbers of people by then. At the time of the Norman conquests, however, these lands were not overpopulated and efforts were made to attract settlers, particularly to improve cultivation. Colonists were found not only from outside the kingdom, but tempted from one region to another. Overall, the lands of the southern kingdom had a well-established reputation for fertility, and contemporaries all took their wealth and prosperity for granted. Although there is now little hope of providing an objective description of the kingdom's prosperity, a high proportion of the thousands of documents surviving from this period if milked for information, create an impression of general economic well-being, despite the occasional occurrence of natural calamities like earthquakes, volcanic eruptions, years of poor harvests and disease.

Unlike the northern Norman kingdom, the lands of the southern monarchy did not form a compact whole. From the Tronto river in the north to the southern tip of Sicily, the distance is about 500 miles; from Marsala in the west of Sicily to Otranto, the most easterly point of the mainland, is nearly 400 miles.