unfortunately if you would use du -h (for human readable) you lose the „sort for size“ mechanism.

you just see what is the largest – but not exactly how large.

# list 30 biggest files or directories (qnap tested)
du -a / | sort -n -r | head -n 30;

# another variant
du -sx /* | sort -rh | head -30;

# if you wanna know the size of a single directory in h-uman readable form you would have to use
du -h --max-depth=0 ./directory

# you could create an alias for that
alias size_of="du -h --max-depth=0 "