There is a system to measuring black holes
Much like anything else, there is a measurement system for a black hole. This system is how a scientist can tell how big or small a black hole is. Schwarzschild created the system, and it is called the Schwarzschild radius.
According to Schwarzschild, the size of a black hole is not determined by the core of the black hole but by the event horizon measured in Schwarzschild radius. This radius refers to the extent to which the escape velocity is equal to the speed of light. Did you learn the equation to this in your physics class?