There is no simple way to measure volcanoes. They are complex and cannot be measured in ways similar to earthquakes or other disasters. One way to measure size would be the volume of lava or ash but there is no quick way to do that. Geologists must visit the site of the eruption and measure the thickness and area covered. Another way to measure how big an eruption is is to use a Volcanic Explosivity Index or VEI. This site has a nice table and explanation.