What is the size and mass of a supermassive black hole ?
It would depend upon the supermassive black hole.
Our ability to even detect black holes is very recent and their nature is mostly conjecture, hypothesis. Regardless, a black hole is measured in "solar mass." Most range from 10 to 100 solar masses. The super black hole at the center of our galaxy is approximately 4.3 million solar masses. -- The solar mass reference is our own sun --