What is the size of a black hole and how does it compare to other celestial objects in the universe?

A black hole is a region in space where gravity is so strong that nothing, not even light, can escape from it. The size of a black hole is determined by its mass, with smaller black holes being about the size of a city and larger ones being millions of times bigger than our sun. Compared to other celestial objects in the universe, black holes can be much smaller or much larger, depending on their mass.