The following command gives me a listing of the sizes of all the directories in the root directory /.
# du -chs /*
Here is the results of that command on one of my servers:
0 /bin 280M /boot 16K /dev 37M /etc 79M /home 0 /lib 0 /lib64 0 /media 35T /mnt 2.8G /opt 0 /proc 7.2M /root 9.4M /run 0 /sbin 0 /srv 0 /sys 324M /tmp 2.1G /usr ...