The size of an earthquake is directly related to the area of the fault plane and how much it moves. The bigger the area and the more it moves, the bigger the earthquake. The largest quakes occur on subduction zones. To imagine the fault plane of a subduction zone, imagine a piece of paper laying on its side. The paper has a large surface area. Faults such as the San Andreas are more like narrow ribbons laying on edge and therefore have a more limited surface area. Although a 'mega-quake' of magnitude 10 is theoretically possible, the probability of such a large quake is extremely small. Since we can calculate how big a fault plane would be required for such a large quake, we can compare this to the known faults of the world. The fact is, there just isn't a fault big enough to make such a quake. It would take 32 of the recent Sumatra 9.0 earthquakes to equal a magnitude 10. The largest earthquake on record was a whopping magnitude 9.5 which occured in Chile on May 22, 1960.