When it comes to grinding balls, shape, sizes, it depends on the grind size required. This is usually determined by what is needed to liberate the desired product from the ore. The ore coarseness and hardness are some of the determining factors. In the wet milling process ball mill operation, we use anything from a mixture of 1" ball or larger. Sometimes ball sizes (mill charge) are mixed with multiple sized ball types. The metallurgist usually does most of the determination based on the ore type at the time. Ball wear is very seldom if ever uniform. The balls will deform or even have a flat side sometimes.
As for a as ball/rod mill plans for a scale or small scale. There are a few nice ball and rod mill small scall examples on you tube that use gallon paint cans with a clear see through lid. I use this method with marbles, large and small with corn flakes or rice crispies to show the example of how the process works in a dry ball mill for training. It also allows one to show how speed effects the process. To slow all the media and ore just rolls, to fast the centrifugal effect happens. With just the right speed one can see the effective impact of the media on the ore at the toe of the mill.
Below link is a good basic overview. Hope not to much detail or to technical.
Hope this helps.