Rousav,
If you're talking about the current config of your AS/400 at the command line enter STRSST,
select "Start a service tool", next
select "Hardware service manager", next
Hit <F6> for "Print configuration"
to exit hit <F3> twice and enter to exit.
If you are talking about minimums for a new system config, see Peoplesoft and an AS/400 vendor to get help sizing a box. Then triple any disk amount they quote, OneWorld or Enterprise One will eat it up faster than you'll learn to control it.