In summary, black holes cause curvature of spacetime based on their mass and distance from an object. They possess only three qualities: mass, charge, and angular momentum, which can be used to calculate other characteristics such as surface gravity. The largest known black holes have been observed to have masses of 18 billion times that of the Sun. It is theoretically possible for a black hole to curve spacetime enough to connect to another galaxy, creating a wormhole, but this has never been observed.
All things with mass or energy cause curvature of spacetime. The amount of curvature depends on the amount of mass/energy and the distance from it. Unfortunately I do not know the math to calculate all that.
Black holes are very special in that the only qualities they possesses are mass, charge, and angular momentum. If you know the value of those three qualities, you can calculate everything else about them, including their surface gravity. For the simplest case, a black hole with no charge and no angular momentum (i.e. non-rotating), the http://en.wikipedia.org/wiki/Surface_gravity#The_Schwarzschild_solution" provides a simple result.
Astronomers have observed black holes with masses of 18 billion times that of the Sun; you can plug that value into the Schwarzschild solution and compute the gravitational "strength" of the largest black holes known.