A black hole's event horizon is the surface-shape around the black hole consisting of the points through which electromagnetic radiation cannot escape from whatever is within. It is considered that there is no way to find out what is within the event horizon other than its mass, rotation, and charge. It can be thought of as the physical size of the black hole, which may be useful for some approximate models, but the weirdness of black holes means there are caveats.
For the simplest black hole (no rotation or charge), the event horizon is at the Schwarzschild radius (2GM/c²) from the black hole's center. Any rotation makes it "wider" (as it would a planet or star), but likely doesn't change the order-of-magnitude.