From my understanding both Toril and Oerth are similar size to Earth . If you also go with the idea that the game rules for a planet all function under the same physics and gravitational rules then I think it’s reasonable to assume Athas has similar gravity and is approximately the same size with similar geologic construction. i.e we don’t muck with the rules if a character from Athas magically appears on Toril. They hit the same, fall the same, jump the same etc.
There are differing opinions on it. I come from the mindset to treat Athas as the about the size and mass of Earth and all the other physics. We’ve discussed it a bit in the geology thread. Here. Planetary Geology of Athas - #45 by Otello
If this is the case that the size is indeed like Earth, there is a whole lot of planet left to explore and define. Is it mostly silt or is there some ocean there somewhere?