There is no rolling ball sculpture without a ball rolling down the track. As long as it is round with smooth surface it will do but some limitations must be considered. If a ball is too light it doesn˙t have enough kinetic energy to perform where kinetic energy is needed and doesn˙t have enough weight to trigger some weight based elements.

The balls can vary in size, weight and material and every rolling ball sculpture is designed for balls with the same weight and size. I never intend to use balls smaller than 25 mm. Ball size used in a rolling ball sculpture depends on observing distance. Greater is the distance larger the balls should be.
Next i´ll describe most common balls used in a rolling ball sculpture.

25 mm glass marbles are the most common ball used in a rolling ball sculpture. There are many colors and patterns to be found and they are quite cheap so they can be easily changed. An ordinary 25 mm (1/2") marble weights around 22 g and that´s enough to be used in all the elements described before. All marbles slightly differ in size, weight and roundness but that mostly doesn´t affect the performance.

35 mm glass marbles are good choice for larger rolling ball sculptures. They are heavier and need thicker tracks and larger frame. They are also more expensive than 25 mm marbles.

Phenolic resin balls are made of same material as billiard balls. They come in different sizes and colors. They have very small tolerance for size and weight and have perfect roundness. Comparing to glass marbles they are a bit more expensive and they lose the shine eventually.

40 mm ping pong balls are a good choice where a low noise rolling ball sculpture is wanted. They are cheap, perfectly round, all the same size, found in every sport shop and in different colors. Down side is their weight and it´s difficult for them to perform loops and some weight based elements.