You could modify the MyFontDialog like this:

public partial class MyFontDialog : Form
{
    private FontListBox _fontListBox;
    private ListBox _fontSizeListBox;

    public MyFontDialog()
    {
        //InitializeComponent();

        _fontListBox = new FontListBox();
        _fontListBox.SelectedIndexChanged += OnfontListBoxSelectedIndexChanged;
        _fontListBox.Size = new Size(200, Height);
        Controls.Add(_fontListBox);

        _fontSizeListBox = new ListBox();
        _fontSizeListBox.Location = new Point(_fontListBox.Width, 0);

        Controls.Add(_fontSizeListBox);
    }

    private void OnfontListBoxSelectedIndexChanged(object sender, EventArgs e)
    {
        _fontSizeListBox.Items.Clear();
        Font font = _fontListBox.SelectedItem as Font;
        if (font != null)
        {
            foreach (FontStyle style in Enum.GetValues(typeof(FontStyle)))
            {
                if (font.FontFamily.IsStyleAvailable(style))
                {
                    _fontSizeListBox.Items.Add(style);
                }
            }
        }
    }
}

It will create a list box aside the font list box with the list of available font styles. As for the size choice, you can simply add a list box with hardcoded list of size: 8,9,10,11,12, 14,16,18,20,22,24,26,28,36,48 and 72, just like the standard FontDialog, since we're dealing with true type fonts.