A fault is formed in the Earth's crust as a brittle response to stress. Generally, the movement of the tectonic plates provides the stress, and rocks at the surface break in response to this. Faults have no particular length scale. If you whack a hand-sample-sized piece of rock with a hammer, the cracks and breakages you make are faults. At the other end of the spectrum, some plate-boundary faults are thousands of kilometers in length.
