Well, the shorter it is, the faster it runs (think speed up, slow down). And, to make it run twice as fast, it needs to be a fourth of it’s initial length.
I know that, and I can explain it, but not as well as Walter Lewin, professor of Physics Emeritus of MIT. If you have an hour or so, and your interested in pendulum theory, you can go to You Tube and check out “For the Love of Physics (Walter Lewin’s Last Lecture).
He explains the oscillation of the pendulum, it’s mass, and the effect that gravity has on it. There are other variables that can vary the oscillation of the pendulum, but this video is a great place to start.
Where else can you get information on pendulums from a 75 year old MIT physics professor while he is swinging from a rope?
Check it out. You can also learn why the sky is blue.