There is a huge range of eruption size, with some affecting areas of just a few hundred metres, whilst others can have devastating impacts across hundreds of kilometres.

 

We typically talk about eruption size using magnitude or VEI. Magnitude is the erupted mass. VEI is the Volcanic Explosivity Index, which is based on multiple qualitative observable criteria, such as column height. Magnitude and VEI follow the same scale and are more-or-less comparable. Eruptions over a magnitude or VEI of 4 are considered to be very large explosive events.

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In general smaller eruptions are much more common than larger eruptions, as you can see from the diagram below, where 'n' is the number of eruptions recorded in each VEI bracket in the last 10,000 years.