The du command display the disk usage.
If you’d like to know the size of a file (or directory),
$ du -h /path/to/file
will print in a human readable (-h) form the result.
You can force it to show the result in Kb or Mb by using the options -k or -m respectively, instead of -h.
If you’d like to know the size of the current folder, without printing all the content, the summary option (-s) will do the job,
$ du -sh
where the human readability option has been added.