Before looking at the geography of the Song of Ice and Fire world, a thorny question should be addressed (even if there is no canonical answer, as yet): how big is this planet anyway?
We know that the action is set on a round planet, as confirmed by Martin and also in The World of Ice and Fire. We also know that Westeros and Essos are located in the northern hemisphere of this planet, and that Westeros very likely is linked to the northern polar icecap either by land or a massive sheet of ice (presumably the Lands of Always Winter, or beyond). The World of Ice and Fire relates multiple attempts by sailors to find a way around a theoretical north coast of Westeros and concluding that it doesn’t exist (or dying in the attempt, anyway). But the question of the size of the planet remains an open one.
An easy, default assumption would be that Westeros and Essos are located on a planet the same size as Earth, since that just makes life easier for everyone. Robert E. Howard’s Conan and Kull stories, Tolkien’s Middle-earth and Robert Jordan’s Wheel of Time books are all set on our Earth in a fictional epoch of history, so we know how big the planet is. Martin’s world is in a different position, with Martin even at one time saying explicitly that the planet is bigger. He even considered making it as big as the world in Jack Vance’s Big Planet books (which is three times the size of Earth!) before relenting and merely saying it was a “little larger” than our world.
This seems to match the study of the climactic zones of the world performed by multiple websites, but most notably in this series of (highly intriguing) articles on the planet’s geological history. Their conclusion is that the planet is approximately 4,297 miles in radius (compared to Earth’s 3,959 miles). This would then work out to approximately 8,594 miles in diameter (assuming Martin’s planet has an equatorial bulge like Earth’s, it’d be a bit more but not a huge amount) and thus 26,998 miles in circumference (compared to Earth’s 24,902 miles), all with an error margin of maybe 100 miles. So based on that, it seems that Martin’s claim that the planet is a “little larger” than Earth is correct: his world is 8.0771% larger than Earth. Well, assuming any of the underlying assumptions stand up, that is.
This means that the map of the Known World in The Lands of Ice and Fire depicts somewhat less than one-quarter of the planet. There’s a heck of a lot of unexplored space out there, in which could be lurking just about anything.
You can take issue with some of these figures – Tor.com, for example, suggests the planet is only 4% larger than Earth – but as a working basis that satisfies both internal logic (the planet is clearly not massively larger than Earth due to issues like gravity, richness in metals and climactic variation) and Martin’s statements, this seems like a reasonable working conclusion.