Many of the definitions in package System are implementation dependent. Furthermore, although the source of the package System is available for inspection, it uses special attributes for parameterizing many of the critical values, so the source is not informative.
The gnatpsys utility is designed to deal with this situation. It is an Ada program that when it runs, dynamically determines the values of all the relevant parameters in System, and prints them out in the form of an Ada source listing for System that shows all the values that are of interest. This output is generated to stdout.
To determine the value of any parameter in package System, simply run gnatpsys with no qualifiers or arguments, and examine the output. This is preferable to consulting documentation, because you know that the values you are getting are the actual ones provided by the system.