All you need to do is collect the terminal velocity of every creature there is, then compare that to how far it has to fall to reach that velocity. Then you have a equation of ant=non-dangerous terminal velocity, so it's "max fall damage"= 0. Elephants would be a different story though. You could also add in a rule for increasing velocity, along the lines of "after 20 ft. You will take 1d10 for every 5 feet(including that first twenty), and modify this again at 50 ft. To a damage of 5d10 for every 5ft., including the first 50."
EDIT: Another one I thought of, for those who like more math, is that you could use percentiles. You take X(+/-)a certain percentage, depending on how far you fell, where X is whatever you roll on the percentile. So you could fall 5 feet, and have a fall damage of percentile roll, where you lose the percentage you roll-20 percent(for example, it could be more of a reduction). Then you could make it impossible to die from a fall less than 15 feet, but entirely possible to die anywhere from 15 feet up, with a guaranteed death at something like 200 feet.