There is a system to measuring black holes
All objects, including black holes, can be measured. Scientists use the measurement system created by Schwarzschild, called the Schwarzschild radius, to determine the size of a black hole.
The size of a black hole, as we know it today, is not determined by its core. Instead, the event horizon – or the extent to which the escape velocity equals the speed of light – is what determines the Schwarzschild radius. Did you retain this information in your physics class?