From A Wiki of Ice and Fire
The Known World is a world composed of at least three continents (Westeros, Essos, and Sothoryos), a large landmass (Ulthos), and a number of many smaller islands.
None of the four major landmasses have been completely mapped. The Known World is a round planet, although it might be a little larger than Earth.
Located in the far west of the known world, Westeros extends southwards from the northern polar icecap for approximately 3,000 miles. The Wall, a colossal fortification which stretches for 300 miles, separates the lands Beyond the Wall from the Seven Kingdoms, a political entity that unifies the rest of the continent. The Seven Kingdoms is divided into nine constituent regions.
Separated from Westeros by the narrow sea, Essos extends eastwards for many thousands of miles. It is larger than Westeros but less densely populated. The western edge of the continent is controlled by the nine Free Cities, while the city-states of Slaver's Bay are located in the south-central region. Much of the continental interior, known as the Dothraki sea, is grassland ruled by the tribal warriors known as the Dothraki. Further east is Qarth and the fabled Jade Sea. The mapped lands of Essos are divided by the Bone Mountains and the Krazaaj Zasqa, which run from the Shivering Sea in the north to the Jade Gates in the south. East of the Bone Mountains are Yi Ti, legendary Asshai, and the foreboding Shadow Lands.
Sothoryos is located south of Essos, on the far side of the Summer Sea. It is a large continent consisting of deserts and jungles. It is said to be a haven for plagues and dangerous animals, and has not been explored much beyond the northern coastal regions. Just north of Sothoryos are the Basilisk Isles.
The largest island in the Known World is Great Moraq, located between the Summer Sea and the Jade Sea to the south of Qarth. The second largest appears to be Ib in the Shivering Sea north of the Kingdoms of the Ifeqevron, followed by the largest of the Summer Isles, Jhala. See also various other islands.
Lord Gylbert Farwynd of the Lonely Light, the island farthest west of the Known World, speaks of lands farther to the west, across the Sunset Sea. This land has no winter and no death. These claims are not treated seriously.
The largest seas appear to be the Shivering Sea, north of Essos, and the Sunset Sea, west of Westeros. The Summer Sea and Jade Sea lie between Essos and Sothoryos, and the narrow sea separates Westeros from Essos.
References and Notes