Pretty simple... if you're doubling the lengths, you're increasing them by 2 times. Take 2 and raise it to the 3rd power. If you're tripling the lengths, you're increasing them 3 times over. Take 3 and raise it to the 3rd power. And so on.
Doubled = 2^3 = 8
Tripled = 3^3 = 27
Quadrupled = 4^3 = 64
Hope this helps
~Ava~