Is there any rule of thumb to calculate the main breaker capacity (amps) of a machine main electrical panel. The panel components are PLC, drive, I/O,... Standard automation panel.
In the US probably the most common way is to figure out the maximum load, add 20-25%, and take the next highest standard size CB.
The trip setting of the CB does not change the price any. That's determined by the frame size, so the tendency is probably to use a CB with the trip setting set at the frame size. Depending on possible future needs you might up size it, or you might want to keep it as small as possible to reduce costs.
There is not even a requirement that there be over current protection inside the box for the incoming feeder.