You can issue the CA SYSVIEW command STORAGE to get the amount of 'Real Storage' assigned to the system.
You will see Real Storage Assigned, Total, and Unassigned. From the Sorage display screen, you can hit the PF1 hey for HELP, and this will provide additional information regarding the STORAGE command.
You can use the Sysview ACTIVITY command to see how much real storage is being used on a system. You can see this on an individual per job basis under the column heading of R-STG. If you want to see the total amount of real storage being used on that system, then you can perform the following command from the Activity display screen:
This command will Total everything in the R-STG column, and will show you the amount of real storage in use.