The most recent evidence is that the 535[536 eruption was the Ilopango Caldera in El Salvador, not Krakatau.
The Icelandic volcanoes, which are going off all the time, can spew Fluorine, and that can roll over an area and destroy all plant and animal life.
That's pretty consistent with the few records of the time in Northern and Western Europe.
What that means is a rather modest volcano belching the right stuff can be more damaging than an enormous continent buster.