Quick Answer: Without knowing details like meridian lines, equators, the curvature of the planet, and without having a fully-realized map of undeveloped continents in Planetos, the true scale of the Game of Thrones can can be hard to quantify in real-world terms. But George R. R. Martin, author of the books upon which Game of Thrones is based, says that Westeros, the continent where most of the story's action takes place, is about the size of South America. The other continents stem from that space.
Ever since Game of Thrones (2011-) first graced television with its massive cast, fiery dragons and terrifying living conditions, fans have been trying to grasp just how big this huge-scale world actually is. The series’ now-famous opening title sequence takes us to all the points where each episode will travel, covering a map which looks and feels as if it’s the same size as Earth. We know Winterfell is in the north, south of The Wall. We know the Lannisters hail from Casterly Rock, where bountiful gold deposits have kept the family rich beyond reason for ages. And at the southernmost tip is Dorne, thousands of miles from the cold. What we don’t quite know is just how big and far apart these places actually are, and where they are in proximity to one another using real-world geographical comparisons as reference.
A shot of the series' opening credits
Characters sometimes require great lengths of time to travel from seemingly-far place to another seemingly-far place. And sometimes they make those same voyages much quicker. This has more to do with plot necessity than logical geography.
It would go without saying to any die-hard Thrones fan, but it’s worth mentioning that Westeros is not the entire Game of Thrones world. It is the western continent of several continents which make up the universe, and the location where much of the series takes place. But there is also Essos, the eastern continent separated from Westeros by the Narrow Sea, and home to the free cities, the Dothraki sea, Braavos, and Pentos, to name a few. There is also Sothoryos, a southern continent hardly mentioned on the series. It is mostly unexplored. Another continent called Ulthos shares that distinction.
George R. R. Martin has said that Westeros is roughly the same size as the continent of South America, north to south. Horizontally, it’s a bit skinnier. And while his word should be gold on how big the land is, that description tends to be in conflict with what he's written in the books.
The novels often use the word “leagues” to describe distance, and on rare occasions the text has qualified a quantity of leagues by saying how many miles it is. For instance, from A Storm of Swords:
“Jon knew the song, though it was strange to hear it here, in a shaggy hide tent beyond the Wall, ten thousand leagues from the red mountains and warm winds of Dorne.”
This is telling us the distance from The Wall to Dorne’s shores is ten thousand leagues. Note that Westeros goes farther north than the wall, so this isn’t the complete vertical distance of Westeros, but a good bit of it.
Also from A Storm of Swords, “I couldn’t even find the Wall. It’s a hundred leagues long and seven hundred feet high and I couldn’t find it!”
And in A Dance With Dragons, we find, “One hundred leagues from Deepwood Motte to Winterfell. Three hundred miles as the raven flies.”
Deepwood Motte is a castle in the forest to the northwest of Winterfell. If one hundred leagues is three hundred miles, a league in GRRM’s universe is equal to about three miles, and Deepwood Motte is 300 miles from Winterfell. Similarly, that puts The Wall at 300 miles long. (A “true” Earth league is 3.48 miles, so this calculation breaks what we think of as a standard measurement.)
Under these assumptions, if it’s ten thousand leagues from The Wall to the shores of Dorne, that distance is about 30,000 miles, which is more than the circumference of our entire Earth. If Martin says the entirety of Westeros is around the size of South America, which is about 6,400 miles from north to south, leagues on a 1:3 scale aren’t a reliable measurement of size. (The books also describe the coast of Dorne as 400 leagues, which would be 1,200 miles, or roughly the distance from Philadelphia to Miami. Obviously that's too big.)
Traveling Thrones style
As it is, the known map of “planetos” lacks scale or curvature. We have no knowledge of meridians on which to place particular landmarks. We don’t know where an equator would be and cannot accurately factor distances on a flat surface. The entirety of the Thrones universe isn’t even mapped as much of it is deemed unexplored. On a basic level, the scale of Martin’s leagues and his intended size for the universe don’t seem to match up, unless a mile in Westeros is far smaller than in the real world.
Martin has also said Essos parallels in size to the lands of Eurasia. Of course, it's basically been proven he doesn’t have a good sense of scale, meaning we can disregard much of what’s written in the books (and, for that matter, what comes out of Martin’s mouth) as calculable evidence. Thus, perhaps it's best to consider most characters' descriptions of distances and sizes as conjecture. We're given the impression most of the Westerosi population is either illiterate or doesn't spend a whole lot of time reading or learning. Education in Westeros is for the elite. Perhaps many of the numbers they spout out are little more than off-the-cuff exaggerations of size, like when you don't go to a certain place because it's "a million miles" from home.
Let's look at a few calculations by others. This one, published on Business Insider, puts Westeros at larger than all of Europe, and sees Essos stretching all the way through China:
The GoT universe laid atop an Earth map, from Sean Garvey via Business Insider
The Huffington Post published this one, where Westeros spans Greenland to Spain:
A map of Westeros from Huffington Post
Here are two infographics put together by The Daily Dot, which shows some of Westeros’ notable monuments against other real-world objects based on descriptions found in the texts:
The Wall vs. huge real-world walls, from The Daily Dot
Heights of things in Game of Thrones vs. the real world, from The Daily Dot. The Eyrie is unfathomably large.
The Daily Dot also puts Westeros’ north-south length at 3,420 miles and its east-west width at 1,410 miles. This is only about half the length of South America. Clearly, everyone is coming up with different calculations.
So what’s the answer? Maybe there isn’t one. It may be impossible to truly grasp the geographic scale of this world. But if Martin says Westeros is supposed to be the size of South America, who are we to argue? Even if the math doesn't always add up within the literature and television show, that's the size the man who created it envisions in his head.