The population estimate for Wales in the early Middle Ages, at the Norman Conquest in 1066, is 150,000. This is squarely in the ‘medieval warming period’ which began around 950 AD, in which Wales experienced a warmer climate than between the 13th and 19th centuries. This site indicates that the population doubled by 1350 to 300,000, but then was cut by 1/3 with the Black Death. It didn’t reach that total again until the 16th century.

As of 2008, the population of Wales was roughly 3 million, creeping slowly up from 2.8 million in 1991.  Cardiff, the capital, is by far the biggest city, with slightly fewer than 300,000 people.   http://www.citypopulation.de/UK-Wales.html.  In the Middle Ages, Cardiff’s population was between 1500 and 2000 people–and was one of the few, and certainly one of the largest–towns in Wales.  http://www.localhistories.org/Cardiff.html

This population is spread over an area of 8018 square miles.  This is roughly the same size as New Jersey (8722 square miles) which has a population of 8.6 million.  In contrast, Umatilla County, Oregon at roughly half that size, has a population of 73,000 and the state of Oregon (96,000 square miles) has 3.8 million.

The vast majority of the increase from then until now came in the 19th century.  In 1801, the population of Wales was just over 587,000; by 1901, it was 2,012,000.   http://en.wikipedia.org/wiki/History_of_Wales