How Big Is the Black Hole?

In the 1930s, Chandra Seikar calculated that white dwarfs have a maximum math. So there are certain classes of heavier stars that could never end up as white dwarfs because they would collect to something more massive than a white dwarf. And it turns out what they collapsed into is a black hole. This was the very first evidence that black holes could exist in reality in the universe. They're kind of an inevitable endpoint for heavy stars. How big does the black hole have to be for us to be able to see it from Earth? How big do we design a telescope that could see it? It turns out there are only two black holes in the sky that are reachable with