DIRSPACE.EXE

    Dirspace is a utility that tells you how much space a directory and
all its subdirectories take on your disk.  It can take a path as an argument
but only a path on the current drive.  If no arguments are given, it assumes
the current directory.  Example usage:

dirspace \windows

This will tell you the number of bytes that the windows directory and all its
subdirectories take.
    Source is included.  A possible improvment would be support for drives
other than the current one.