`du -h --max-depth=1 / | sort -h -r`

This will show each folder in `/` including `/` itself.

 * `-h` options shows sizes in human friendly format
 * `--max-depth=1` instructs command to go only 1 directory deep inside `/`
 * `sort -h -r` sorts results using human friendly sizes and `-r` instructs command to show results in reverse order (from largest to smallest directories)