How big (in terms of volume) a black hole is, depends on what you define as belonging to the black hole. If you define the boundary of the black hole as its Schwarzschild radius, then it can cover a fair amount of volume. The black hole at the centre of our own galaxy, in Sagittarius, is estimated to have a Schwarzschild radius of about 1.33 x 10
10m, which is certainly tiny compared to the whole galaxy, but is very far from being a point or singularity.
http://en.wikipedia.org/wiki/Schwarzschild_radius
How extremely stupid not to have thought of that - T.H. Huxley