Actually df -h gives the disk space in a human readable size. This means it will translate it to KB, MB, GB, TB, PB all after how big the filesystem is. Notice that the -h flag outputs the size with the powers of 1024. This means M stands for 1048576 instead of 1000000. If you want the offisial SI units, use -H or --si.
du, df, ls and find are all great tools for monitoring disk space usage. man <command> will give you the manual on how to use them.