Cycling makes you stretch your legs to its maximum and then some more, thus stretching the bones, tendons, ligaments and muscles. Extended periods of these stretches will ultimately lead to lengthening of the legs.
Proper cycling for lengthening your legs.
In order for proper leg lengthening by cycling, you will need to raise the seat of your bike higher than normal. Raise your seat so that the balls of your feet are barely able to touch the pedals. It may be a bit more difficult at first to ride your bike in this manner, but doing this is one of the best ways to force your legs to stretch and lengthen.
For safety, you may want to experiment with this on a stationary exercise bike first. Cycle for 15 ~ 20 minutes every other day (3 times per week) at a moderate pace.