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Dimensions of the Planet Mystaraby Ann Dupuis
>>Q: What is the radius of Mystara?<<
Mystara is 6,190 miles in diameter (3,095 radius).
Its circumference at the equator is ~19,400 miles.
The crust is ~ 1,200 miles thick.
Its two polar openings measure 1,500 miles in diameter at Mystara's outer surface (they narrow as they approach the World Shield, then widen again to 1,000 miles in diameter within the Hollow World).
The total surface area of the planet's outer world, minus the polar openings, is just over 105 million square miles. 60% is covered by water.
The interior diameter is 3,790 miles (measured from any point on the Hollow World's surface through the centre of its floating sun to the corresponding point on the opposite side). The interior circumference at the equator is 11,908 miles. The Hollow World has a surface area of just over 38 million square miles, 70% covered by water.
And yes, Mystara is smaller than the Earth. Gravity, being generated primarily by the World Shield, is the same as Earth's, despite Mystara being hollow. (It's the same within the Hollow World as well as the outer world. It gets tricky in the caverns that actually penetrate the World Shield, including at the capital of the Shadow Elf realms.)
Hope this helps.
(Oh, and if anyone's curious, Earth is 12,756 miles in diameter at the equator, for comparison. A lot bigger than Mystara.)