AIX:
As a root user, you can run the following:
bootinfo -s
The result should be in KBytes
If the physical volume has been allocated into a VG, you can use the following command (it does not need root privilege):
lspv
Use command by any user:
df -lh
result
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 114G 87G 22G 81% /
/dev/sda1 99M 15M 79M 16% /boot
none 1007M 0 1007M 0% /dev/shm