For siemens CPU, you have "work memory" which is non expandable memory and is the limit of what your CPU can do.
ET200S CPU like the IM151-8 have 192 KO of work memory. S7 300 goes from 384 to 2000, S7 1200 from 25 to 150.
The size in memory depends of a lot of things: complexity of the machine with communications, diagnostics, number of devices and the programm (string manipulation, complex structures, number of data...).
Generally, the ET200S CPU is too small to control a line of several workstations or machines but can control one single machine or workstation.