GetSize

public static long GetSize(this DirectoryInfo dir)
{
    long length = 0;
    
    // Loop through files and keep adding their size
    foreach (FileInfo nextfile in dir.GetFiles())
        length += nextfile.Exists ? nextfile.Length : 0;

    // Loop through subdirectories and keep adding their size
    foreach (DirectoryInfo nextdir in dir.GetDirectories())
        length += nextdir.Exists ? nextdir.GetSize() : 0;

    return length;
}

DirectoryInfo WindowsDir = new DirectoryInfo(@"C:\WINDOWS");
long WindowsSize = WindowsDir.GetSize();