Back in the days when men were real men, women were real women, and small, furry creatures from Alpha Centauri were real small furry creatures from Alpha Centaury, an obscenely rich woman decided she wanted to have the biggest diamond of all.
Realizing that this would mean a diamond of planetary size, she went to Magrathea and ordered it there. She had only two conditions: It should be a perfect single-crystalline diamond, and it should be impossible to make a larger one, because trying to do so would make it stop being a perfect diamond. Of course, Magrathea delivered what was ordered.
Now as long as the diamond planet was in the hyperspace construction halls of Magrathea, all natural laws could be effectively put out of force as needed, but as soon as the planet was put into normal space, it was under the full force of the laws of physics.
Now my question: How large was the diamond which was ultimately delivered by Magrathea?