A cube is a shape that the length = width = depth. To calculate the volume, you multiply these together, so if L is the length (and therefore the width and depth), the volume is L x L x L, or L3.
To go from the volume to the length, you take the cube-root of the number. The cube root of 6 billion is hard to do without a calculator, but I can give you this approximation:
Since 6 billion = 6 x 1 000 x 1 000 x 1 000, then the cube root of 6 billion is 1 000 times the cube root of 6. That's easier to solve. The cube root of six is a little less than 2 (because 2x2x2 is 8 - too much). So the answer is a bit less than 2000.
6 billion is about the number of people living now on Earth. If you gave each person a box that was 1 meter by 1 meter by 1 meter to sit in and then stacked them into a big cube, you'd have all the people in the world sitting in boxes in a space that is a bit less than 2 km by 2 km by 2 km!
Stephen La Rocque.>
PS: (I finally found a calculator and it says the cube root of 6 is about 1.817, so the cube root of 6 billion is one thousand times that.)