Tsunami warning center scientists usually measure an earthquake's "size" with the moment magnitude scale rather than the older but more famous Richter magnitude scale.
The moment magnitude scale is better suited for measuring the "sizes" of very large earthquakes and its values are proportional to an earthquake's total energy release, making this measurement more useful for tsunami forecasting.
Moment magnitude numbers scale such that that energy release increases by a factor of about 32 for each whole magnitude number. For example, magnitude 6 releases about 32 times as much energy as magnitude 5, magnitude 7 about 32 times as much as magnitude 6, and so on.
This animation graphically compares the relative "sizes" of some 20th and 21st century earthquakes by their moment magnitudes (according to USGS/NEIC). Each circle's area indicates its relative energy release, its color indicates its tsunami potential (see http://ptwc.weather.gov), and its label indicates its moment magnitude, its location, and the year it happened.
Cosmological forecast earthquakes in April 2014. Grand Cross of the planets.