Officially, there is no map of the entire planet. Unofficially, there are several floating around the internet.
What maps do exist?
The one from the original 2e boxed set, which is (I think) the same area covered by the one from the 4e campaign setting book, which is the Tablelands and the immediate surrounding area.
The one from Valley of Dust and Fire, which showed a good chunk of the Silt Sea, the titular Valley of Dust and Fire, and a tiny bit of the eastern shorline of the Silt Sea.
The one from the revised 2e boxed set, which mostly expanded northwards to places like the Last Sea, but also a little southwards to show the northern chunk of the Dead Lands and westwards to show the Crimson Savanna.
I think there’s one from Windriders of the Jagged Cliffs, which mostly expanded northwestwards to show even more of the Crimson Savanna, but I’m not positive.
Lastly, I’ve heard there was/is/will be one included with Secrets of the Dead Lands that will expand southwards to include most/all of the titular Dead Lands. This was a 2e product that was never released. Athas.org has been working on updating it to 3.5e, and last I heard they were really close, but nothing as of yet.
As for the rest of the planet, there have been a few descriptive hints.
The original 2e boxed set actually describes how big the Silt Sea is, and that the Ringing Mountains surround the entire thing. Along the western and eastern coasts, the mountain ranges run parallel to the coast, while along the the northern and southern coasts they run perpendicular. Later maps, however, show the Ringing Mountains to stop along the boundaries of the original map, so take that as you will.
Lastly, the 4e campaign setting book states that the rest of Athas is actually worse than what has been mapped/described thus far.
I hope that helps.