Question: What was the biggest volcanic eruption in history?
Answer: History only gets you back so far, but let's see. The biggest eruption since the rise of science, let's say 1800, would be Tambora in 1815. The biggest one we have direct written records of would be Santorini, around 1630 BCE. However, the Taupo eruption of New Zealand in the year 186 was larger—only no one lived there at the time.
But the really huge ones were long before those. The largest since the rise of modern humans, Homo sapiens, is the great eruption of Toba, in Sumatra, about 71,000 years ago. It produced some 2800 cubic kilometers of ash and may have reduced the world's human population to only 10,000 people (here's a discussion).
The largest eruption we have firm evidence for in geologic history is the La Garita Caldera eruption in Colorado, about 28 million years ago, in the Oligocene Epoch. That put 5000 cubic kilometers of lava into play. There might be larger ones, but the farther back in time we go, the more geologic evidence is destroyed by tectonic activity. Here's a list of the largest calderas on Earth.
