You are correct in saying that black holes are measured by their mass not their volume. Black holes can have a measurable circumference but the radius, (and therefore the diameter) is infinite.
If the radius and diameter are infinite then so is the circumference.
The "radius" of a black hole is generally reckoned as the event horizon. That is finite. If it were infinite the entire observable universe would fit into each and every black hole. The gravitational field extends infinitely in all directions.