In the vacuum of space pairs of virtual particles, consisting of a partial and antiparticle, come into existence by tiny quantum fluctuations in space. Normally the particle and antiparticle annihilate each other in an instant. If a pair of these particles and antiparticles come into existence adjacent to the event horizon and one of the members is pulled in to the black hole, the other member could escape the black hole because of its relativistic speed provided it is still on our side of the event horizon. One particle gets striped away from the virtual pair and gets sucked into the black hole while the other particle radiates away into space. This is called Hawking radiation.
Black holes come in sizes from stellar to galactic.
As the mass of a black hole increases so does its size. A black hole with the mass of the Sun would be a few kilometers in size. A black hole with the mass of the Earth would be the size of a golf ball. In falling antimater from Hawking radiation evaporates away black holes. A black hole the size of a solar mass will decay in 10 to the 65th years.