Please Help - Listing Directory With Directory Size

FolderBrowserDialog folderBrowerDialog = new FolderBrowserDialog();
folderBrowerDialog.ShowDialog();
string selectedPath = folderBrowerDialog.SelectedPath;
string[] selectedDirectories = Directory.GetDirectories(selectedPath);

long size = 0;

foreach (var dir in selectedDirectories)

{
        string[] files = Directory.GetFiles(dir);

foreach (var file in files)

{

        FileInfo fileInfo = new FileInfo(file);

        size += fileInfo.Length;

}

}

listBox1.Items.Add(size);