In Crimes of Grindelwald, Dumbledore casually casts a spell which creates thick fog across London:



The screenplay written by JKR has the same content:

This reminds me slightly of Deathly Hallows, where Flitwick casts spells over the entirety of Hogwarts:

In both cases we have spells which:

• Have a large area of effect, likely in the range of several square miles.
• Are cast with zero preparation or implements other than a wand.
• Are cast as ordinary spells, with a normal incantation.
• Take effect immediately.

What do you make of this? Do we simply have to modify our understanding of the effective range of HP spells, such that casting spells across a large area is a perfectly achievable feat?

Both Dumbledore and Flitwick are advanced magic users, so it likely is not something any old wizard can do, but their casting is quite casual, not depicted as any kind of special accomplishment, so it may not be limited to only wizards of their level.

To be fair, we mostly see magic through the lens of unqualified wizards still learning about magic, not adult users, so we really have no idea of what scale of magic is considered normal for adult wizards.

Further evidence that large scale magic is fairly normal would be Fantastic Beasts, where individual aurors are able to repair entire skyscrapers using a single Reparo.

 

There's Voldemort's name taboo, which is a sound detection spell that apparently affects all of Great Britain.

Then there's the Fidelius charm, which you could argue is constrained to a small location, but seems to affect people's memories everywhere.

Granted, these could be considered "conceptual" spells, rather than physical ones.

 

The idea that simple spells can be scaled up nearly infinitely, rather than requiring a brand new spell that has the same effect but larger is interesting. It seems to be a mostly movie based concept, and it echos the Star Wars idea that size matters not. If so, it might be a matter of belief or concentration or something like that. Could a first year fix a skyscraper with one spell? Probably not, but what is the mechanical difference between their reparo and an auror's reparo? Could Dumbledore wave his wand and reconstruct a city? I think that part of it is that the Statute of Secrecy, and advances in muggle surveillance mean that spells on that scale would be discouraged, and probably would result in jail time or a fine depending on how publically they were performed.