Well there is this, "The map of the sea floor, made by the ship’s multibeam sonar, indicates that as much as 5 cubic kilometers of magma erupted onto the sea floor."
I think 5 km^3 is a large eruption by any standards. Looking at lists of modern ones it's a index 6 event which happen every few decades on land. From the article it sounds like if it weren't close to Mayotte it wouldn't have been picked up seismically.
Is this the largest ever because we're just starting to be able to map this type of stuff or have we seen a lot of them and this is enormous?