``I heard that earthquake was so big it broke the Richter Scale!'' Believe it or not, I once had someone call me and tell me that. Of course, if you guys had been around, you'd have been able to tell the caller that the Richter Scale is not a physical thing, and thus can't be broken.
However, the caller's comment brings up an interesting question: how big can an earthquake get? Unfortunately, I can't give you a simple answer here. The maximum magnitude earthquake that a given fault can generate is determined by a number of factors. A long fault can generate a larger earthquake than a short fault, all other things being equal. A fault which tends to break and have the rocks jump farther than another fault will tend to generate larger earthquakes than that other fault. And the strength of the rocks is another factor: stronger rocks will tend to hold out longer, and generally break in a larger earthquake, than will weaker rocks.
I'm not sure how big the theoretical maximum size earthquake is period, but I can tell you that the largest earthquake ever recorded was in Chile, on 22 May 1960. It had a moment magnitude of 9.6, broke an area of fault 850 kilometers long and more than 120 kilometers wide, and generated a lot of damage and a humungous (taller than 30 feet in some places along the Chilean coast) tsunami. This earthquake released about as much energy as would be released by blowing up one billion tons of TNT!
At the other end of things, there is no limit to how small earthquakes can get. The instrumental limit is that, in the quietest locations with the most sensitive seismometers and earthquakes extremely near to the receiver, it is possible to record earthquakes as small as magnitude -2. An earthquake that small would rupture a circular fault roughly 7 centimeters across and move it about 1 centimeter -- a tiny, tiny, tiny earthquake.
People generally stop feeling earthquakes when they drop below about magnitude 3 or so, although I know of a case where a magnitude 2.3 earthquake was felt by someone sitting very quietly in a house which was right on top of the epicenter.
Just as an aside, there was a great earthquake (magnitude 8.2) in Bolivia in 1994 at a depth of about 630 kilometers which was actually felt in North America. The earthquake was felt in high-rise buildings as far away as Renton, Washington -- which is almost 8700 kilometers from the epicenter! This is the greatest distance over which any earthquake is known to have been felt anywhere in the world.