Important physics fun fact (inspired by some art I just saw). A black hole's radius is proportional to its mass. The visible black shadow of a black hole is about 2.5x its radius. A rule of thumb works out nicely to: One inch of visual radius ≈ one Earth mass. Most black holes you could hold in your hand (however briefly, while you still have a hand... and a body... and a planet) have surface acceleration in the tens to hundreds of *quadrillions* of g's.