The most powerful volcanic eruption in recorded history was the 1815 eruption of Mount Tambora, an explosion equivalent to 33 billion tons of TNT.
Is there any way a cryovolcanic eruption could be as large as this? I couldn't find a source on how powerful a typical eruption is, but based on the low temperatures, I can guess that they're small. However, in a hypothetical moon where there is much stronger tidal friction than we see in the Solar System, I think the eruptions can become much larger.