How big is a black hole? Black holes come in many sizes and their size depends on how much material is in them (their mass). Some are the remains of a giant star which collapsed. A star has to be much more massive than our Sun to become a black hole. These types of black holes are only a few miles across. Black holes have also been discovered in the centers of some galaxies. These black holes are very large and contain the same amount of material as 100 million or more suns. These types of black holes are several million miles across.