We can say how much mass it has, or we can say how much space it takes up. Let's talk first about the masses of black holes. There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density. We suspect that most of the black holes that are actually out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star.
Astronomers also suspect that many galaxies have extremely massive black holes at their centers.
The more massive a black hole is, the more space it takes up. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometers. Three million kilometers may sound like a lot, but it's actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000 kilometers, and so that super-massive black hole has a radius only about four times bigger than the Sun.