@fattony12000: True. But is also says: Historically, there is no physical evidence for the existence of this "katana like sword legendarily used by ninja", though it is believed that they are based on the design of the wakizashi or chokutō type swords. Dr. Stephen Turnbull, a historian specializing in the military history of Japan indicates of historical ninja: "The most important ninja weapon was his sword. This was the standard Japanese fighting sword or katana...for convenience the ninja would choose a blade that was shorter and straighter than usual".
So just by using logic and although there is no physical proof, ninjas had to use a shorter sword if they wanted to unsheat it.