What is the physical size of a black hole?

A few comments:

1) We don't know what happens with objects of volume smaller than $\ell_{p}^{3}$, where $\ell_{p}$ is the Planck length, which is approximately $10^{-35}$ m. The known laws of physics do not hold for anything that small.

2) General relativity, in its classical glory, predicts that objects will collapse down to a shape with zero volume. A spinning black hole will take the shape of a ring, with a radius determined by the angular momentum and mass of the black hole. Since it is virtually impossible to tune down the angular momentum of physical objects to exactly zero, we will expect most singularities to be ring-shaped, perhaps having a $\ell_{p}$ thickness.

3) the singularity is not expected to be observable to outside observers, so most astrophysicists will instead talk about the size of the black hole horizon, which is a macroscopic size.

Usually the size of the black hole is considered to be the point known as the Schwarzschild radius. This is the point at which time slows down to the extent that matter is never seen crossing it - one becomes frozen in time approaching this point. It is given by $2 G M/c^2$. As you can see the black hole can in principle be as small or as large as one desires, although interactions other than gravitational might prevent very tiny black holes from forming (it is an interesting question what is the smallest possible size of a black hole, to think of it). I suggest you read more about it here: http://en.wikipedia.org/wiki/Schwarzschild_radius