In the Seventh Day Adventist Church*, there is no shortage of making connections to real historical events and the seals and other events in Revelation. Particular to your question, the SDA church has made connections to real earthquakes (and other "cosmic events") and a few of the seals. With that, we can infer "how big" the earthquake should be, or rather, was.
Adventists have an historic view of eschatology. That means that they think the book of Revelation is a prophetic record of the entire history of the Church from its writing to the Second Coming of Christ. They also believe that seals represent an era in Christian history; they are not exact, shortly executed events (typically). So they do have some opinions on actual historical earthquakes being events that Revelation was indeed talking about. These events are often quoted to show that a certain point in history was in a particular seal era.see video Source 1
Among them are the 1755 Lisbon Earthquake, New England's Dark Day of 1780, and A meteor shower in 1833. [Wikipedia source] see video Source 2
Now the Lisbon Earthquake is estimated at about 8.7 magnitude. The Lisbon Earthquake, by measure of how many people and lands were affected, was certainly no small event; countless people were affeted by it and tens of thousands perished because of it. It was a massive earthquake no matter where you live and what you are used to. With this in mind, you might infer that the SDA would say that a "massive earthquake" should be in this range. However, to SDA's, it is not necessarily the severity of the event itself that signal the event as a "cosmic sign".
A "cosmic sign" is better recognized by people's reaction to it. In general, they look to see if the event caused a turning to God (out of fear). The Adventists quickly recognize that the time frame they mark as the era of the 6th seal was also a time of heightened religious interest. Some of those eras are called by secular historians The Enlightenment and also the Great Awakening. see video Source 4 & 6
I was lucky enough to find this lecture on YouTube that supports what I have said about SDA doctrine and belief.
Starting about 14 minutes in: "The seals are a countdown to God [The Second Comming]". He describes the seals as eras.
Starting about 17 minutes in: He Justifies that the 6th seal cannot be an instanteaneou event.
Starting about 20 minutes in: The speaker just finished reminding the listeners that the "time, times, and half a time" from Daniel is the Dark Ages Church, which they say officailly ended its reign in 1798 (according to SDA's) then says that they should begin seeing the signs described in the sixth seal. He then explicitly states the Lisbon Earthquake of 1755 as on of the signs.
Starting about 25 minutes in: Mentions that people ran to the Churches for protection during the Lisbon Earthquake, but perished anyway. He then begins to discuss New England's Dark Day.
Starting about 30 minutes in: He beings to discuss the Lionid Showers in 1833.
Starting about 32 minutes in: He says that the opening of the 6th seal was a time during which people's religious interests were excited and awakened (The Enlightment). Also, he states explicitly that the 6th seal was not an instantaneous event.
Starting about 35 minutes in: He mentions an earthquake in Chile (thought you might be personally interested).
Additional lecture source
*Disclaimer: The SDA is quite unique in a great deal of it's doctrines, especially eschatology. The greater portion of Christianity would either disagree with the above or weigh in no opinion at all.