They've disproved that blackholes holds galaxies together. Stars hold their system together, but blackholes do not hold galaxies together, that is done with dark matter.
From what I understand most (but not all) galaxies have a massive black hole in the center. Scientists just photo'd the one in the middle of the Milky Way called Sagittarius A*. It's 17 times bigger than our sun (by diameter). The largest blackhole, M87 is 6.7 Billion times larger and would encompass our whole solar system.
The largest black hole is 11 times the diameter of our solar system. I think it's called a16 or something.