I. 7th and 8th Centuries. Settlements changed a lot during the early middle ages. Moving into the Viking Age, most Scandinavian settlements in the early 700s were either small rural villages or temporary trading sites. These temporary trading sites were located on open coastline, where merchants could easily case the premises before landing and quickly disperse in case of a threat. Craftsmen and merchants would set up tents, perhaps for as long as a few months, then move on to the next site. Some of these may have been managed by upstart jarls or aspiring kings who would partition regular plots of land for craftsmen to use. Lundeborg in Denmark is a good example. It was the port for a local religious and (probably) political center called Gudme. I haven't found any reconstructions of Gudme, but Uppåkra in southern Sweden is a similar site: a very open settlement dominated by long houses and smaller out buildings, perhaps with some sort of defensive palisade surrounding the entire perimeter.

II. 8th and 9th Centuries. In the late 700s and into the 800s, a new set of urban centers emerged. These often developed at natural borders, such as at Åhus in southeast Sweden where merchants would stop for a rest before entering the more dangerous and pirate-infested waters to the north around the islands of Öland and Gotland. These sites were often a bit harder to get to, somewhat upstream from a river mouth or deep in an estuary or fjord. This gave merchants a greater sense of security that allowed them to establish more permanent dwellings.

Birka in Sweden (a bit further inland than modern Stockholm) is an excellent example. It might've had a population of around 600. Most people (i.e. women and children) were involved in making textiles, which were necessary for clothes and sails, and there were some small industries as well. My impression from the texts and archaeology of Birka is that it was a market where merchants from the far north would bring furs and walrus ivory, raiders from the west would bring Anglo-Saxon or Frankish slaves, and Islamic traders would bring silver and beads. There was an earth wall with a simple wooden palisade, some outlying obstacles to prevent surprise attack from the sea, and a small and sparsely provisioned hill fort just in case worse came to worst (which might've happened every 50 years or so). The elite liked to be buried with their weapons, which were badges of honor, but in most of the burial mounds that surround the walls, the majority people were buried with trade or craft goods. This was not a warrior society.

III. 10th and 11th Centuries. While magnates may have been interested in these early trading settlements mostly as a way to get exotic luxury goods, later kings saw them as an opportunity to milk taxes. They invested more of their resources into building infrastructure, and they protected their investment by developing urban centers at more secure sites. This change began in the late 900s and was significantly reshaping Scandinavia by the time the Viking Age ended in the mid-1000s.

Southern Sweden, which at this time belonged to the Danish monarchy, is one good example. As mentioned above, a pre-Christian temple had a long existence at Uppåkra. Around 1030 the Danish king started to develop a new royal and ecclesiastical site at Lund to replace it. He brought in English money makers to establish a mint, which is a good sign that he was using Lund as a tax hub. He brought in a bishop that had earlier served him in the Orkneys north of Britain, making it an important religious center as well. And a later bishop brought in stone masons from Germany to lay the foundations for the modern cathedral. Lund was probably a very cosmopolitan and wealthy place, and it probably had a harbor town (just like Gudme had Lundeborg), but these can be hard to find and prove through archaeology.

Overall. From more of an impressionistic perspective, towns were not particularly pretty places, and their populations probably numbered only 300-1000 or so. Merchants and their families might've had better access to goods like clothing and jewelry than their more humble neighbors living on nearby farms, but their quality of life suffered from disease and the dangers of travel. Life expectancy was perhaps not even 35, and most women were dead and buried by their early 20s, so these were very young communities indeed. Children would have been constantly spinning yarn, women weaving textiles, and men frantically peddling their wares in an effort to feed their families and buy wood for winter fires. It's little surprise that the super elite of the Viking Age often chose to live elsewhere, if the king hadn't sent them in as a taxman or a bishop.