General relativity says that mass distorts space and time and the more mass, the greater the distortion of space.
A black hole is formed when a mass causes space and time to distort so much that nothing that enters the black hole can ever get out.
It takes a lot of mass, much more than the mass of our sun, to make a black hole.
We can detect black holes because we can observe the effect of the distortion of space around the black hole.
Their "size" of a black hole the region of space called the event horizon. If anything, light or matter, enter the even horizon, then it never comes out. In effect, the size of the back hole is the region of space insize that can't be seen.