In the game Riven, on the island with the large gold dome, one can find a telescope which points into the ground. If opened, one can see stars, which suggests that perhaps a lot of the ground beneath is either gone or never existed
That telescope points into the Star Fissure, it's unclear what the star fissure is, other than:
Things that fall into it end up on Earth. It can be traversed by people (though perhaps not safely). The star fissure isn't exactly an indication of parts of the age going missing, but more likely an expanding inter-dimensional rift of some sort.
It's generally accepted that an age is a complete world or planet itself. The books mention that sometimes an age is destroyed when for example the world has fallen into a star (as mentioned in the Book of T'ana). There are counters to this rule however such as Catherine's "impossible" age. Or the prison age written for Ghen.
The fissure represents a break down in reality. Whatever the shape of the planet (if it even is a planet) the fissure is just a literal hole in space-time physics.
The possibilities for shape of the planet and what else lies on the planet are limited only by what Gehn wrote into Riven. That's the way The Art works. Any conceivable (or inconceivable) reality that would contain the details laid out in the descriptive book could work. Riven could be inside a giant space ship, or a ring world. It could even be a inside-out sphere world (which would explain the fissure quite well actually.) Occam's razor would dictate that world is most likely round, and there are many land masses on which the creatures of Riven evolved.
