Every realm has a limited amount of habitable space. Inevitably, there is a barrier to travel on the border of the realm. In Muir, for example, this barrier manifests as an acidic bog which is treacherous and filled with dangerous creatures.
Realm borders generally fall into 3 categories: oblique, impassable, and infinite.
Oblique borders are circular in nature, like a globe. . A traveler leaving to the west and traveling for several days might find herself leaving the border from the east, having circumscribed the realm.
Impassable borders prove difficult or impossible to pass. The Claws of Man for example, has an infinite abyssal cliff stretching down from its entrance. These realms are frequently used by the Adversary as popular means to execute criminals and dissidents. These executions draw large crowds from nearby realms, as they are generally gruesome and unusual.
Finally, some realms appear to stretch on infinitely. Cedarhold is a large island surrounded by an ocean that stretches on forever. Brave souls have attempted to find other islands in the ocean, but have never been successful.