The topic is summarised nicely by J.B.S. Haldane in his essay On Being the Right Size
You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes.

This assertion depends on air resistance. Drag acts against the force of gravity on a falling object; the object reaches terminal velocity when the force of gravity is equal to the drag. Obviously the smaller the terminal velocity, the better the chance of a happy landing. Very roughly the terminal velocity is proportional to the size of the falling object. So the terminal velocity of a mouse is approximately 1% of that of an elephant. To give specific examples, a parachutist falls gently to earth at around 5 m/s whereas a bacterium is travelling at 2.8 x 10-4 m/s.

In a study done by some New York city vets (where apparently it is quite common for cats to be defenestrated) it was found that it takes a cat about 7 floors to hit terminal velocity and that a cat can often survive this with nothing worse than a nosebleed.

Some facts were sourced from
• http://www.pls.lib.ca.us/smcccd/faculty/goth/MainPages/Scaling/Scaling.htm
• the rats-digest mailing list at the University of Washington,
• http://ffden-2.phys.uaf.edu/211.web.stuff/Kuhns/variables.htm, and