Well according to this:
....Two Black holes in a tango...eject stars from the center of elliptical galaxies....I guess so that one of the black holes can't get bigger than the other one...
But there is no evidence for a Central Authority seen by the Hubble that tells a Black Hole ...You Must stop eating Now.!!!
Dark energy is an intriguing thing.
as to a black hole and size it can get, the sky is the limit. ;-)