Good question! Scientists have estimated the solar flare that occurred prior to the Carrington event might have at around an X-45 (Elvidge et al, 2018). I believe that this was the first flare ever recorded, but as you mentioned, we obviously we didn’t have modern detectors back then for measurement.

The theoretical limit for a solar flare is very high. These are called super flares. However, they are exceptionally rare and tend to occur in younger, larger stars than our sun. That said, it isn’t impossible for one to occur, and if a massive flare were to occur, the flare wouldn’t be the worst part. Simply put, a solar flare is magnetic energy being launched from the surface of the sun in the form of photons or particles of light. While this would create radio blackouts, satellite drag, and phenomenal aurora, it wouldn’t have a significant impact on Earth’s surface (except for the satellite issues). The real danger is the coronal mass ejection that would follow. A coronal mass ejection is protons, neutrons, alpha particles, and electrons being launched at insane speeds off the Sun’s surface. Because these particles are charged, they can impact our electric grid. And in an extreme case could, in theory, destroy it through massive electrical ground level enhancements. These enhancements would primary occur at higher latitudes because the charged particles would follow Earth’s magnetic field lines toward the poles, but could easily disrupt electric grids northward/southward of 30 degrees. That would mean most of Europe, America, much of Asia, Australia, half of South America and Africa all without power. Obvious chaos would ensue. The good news is governments are aware of this risk and have begun taking preventive mechanisms to prevent such a catastrophic failure. The other piece of good news is this type of event is very rare. The likelihood of it happening in anyone’s given lifetime is extremely low.