To have a Black Hole first you must decide that gravity always is the same no matter how strong it gets.

If this is true or not is only a guess, but it is just as hard to prove it is not true as it is to prove it is.
After all no one knows what gravity is in the first place.

But, if you decide that gravity always acts the exact same way as it does where we have been able to measure it then you can wonder what will happen if you shrink a mass down to a very small space.
If you shrink something small enough force caused by gravity may become strong enough to cause light to follow a path which looks like a circle from someone watching at a distance.

A very large mass would not have to be shrunk as much to get very strong gravity and the light would circle in a bigger circle. A smaller object would have to be shrunk a lot before it would have strong enough gravity so the light would have a much smaller circle.
So Black Holes may have different sizes.

If gravity gets strong enough to cause light to circle and fall back nothing can get away, not even light. If this happens it would be a black hole.

Black Holes are a very emotional idea for many of the people who think they are smart and right messages here. They may want there to be Black Holes very badly or they may not want there to be Black Holes very badly. Sometimes they argue.

The fun part is you may be able to guess as well as any of the people who write messages here.

I hope this helps.

Duane