[crz]

Mount St Helens was a VEI 5 or there abouts. A VEI 6 is ten times largerthan the prior. So if St Helens is one foot, then the VEI 6 is ten feet. A VEI seven is 100 feet. A VEI 8 is 1000 feet. Just to put it in perspective.
Then comes the duration. Two days? Four? One week? Two weeks?

Yellowstone was the second largest known behind Toba. The third or fourth? The Long Valley eruption..And that one has the magma ABOVE the valley floor at this time in the mountains around it.

[palmer]

Lake Toba is 440 square miles. Lake Pinatubo is 0.7 square miles. Lake Taal is 94 square miles but most of that lake was created 100k to 500k years ago and that is not likely to repeat.

But I think you are right about looking at duration. The 1754 eruption lasted 7 months. That could have unpredictable effects like the Icelandic volcano did to Europe.