#Reality check
Reality check
If we make Haark the size of Australia (Approx. 3.3m square miles) and give a decently high population density 500 people per square mile (approx. density of all of the UK). You would have a population of 1.65b people. This is a bit over 1/3 of the population of Asia (4.5b approx.). China has 1.4B approx. at 3.7m sq. miles for comparison. If the amount of people living on shard is high enough it would be possible to have a city that covers the entirety of it as the people would need that space.
#Urban Sprawl
Urban Sprawl
Urban sprawl is the expansion of human population into low-density communities. This would be your suburban neighborhoods. If we consider these part of the metropolis the reality of squeezing every inch of land into the definition of city is a lot closer.
#Building Limits
Building Limits
If we stick with medieval buildings, we would have significantly shorter buildings. This would lower our population density maximum and would contribute to the urban sprawl.
#Yes
Yes
Having a continent sized city is plausible given enough population density. However, I don't know what the true scale of what size they think the shards should be. The only clear clues that I think we have at the moment are those pictures. I would assume from them that having a metropolis of the size pictured there wouldn't have much issue from a believability standpoint.
I don't think this really answer the question - it only says that the specific Haark example might be plausible at real-world scale (which I also don't think I agree with).
The two main issues presented are: Haark for instance, doesn't seem plausible at continent scale (argued above) AND What is the actual scale which I said based on the images seem quite compact in comparison to a continent. Thus, I argue that Haark at real-world continent sizes works and that the intent of the text seems to state real world size while the art shows it much smaller.
Bonus Fact: Population density of some countries reaches higher than 20,000 per square mile and I compared at 500.