Learn to create Python3 plugins

[z-uo]

I try to use Glade and it seams work correctly, But I do not Understand why it expand automatically on the width but not on the height:

Code:

GimpUi.init("add_balloon.py")
dialog = GimpUi.Dialog(use_header_bar=True,
                                   title=_("Add Balloon"),
                                   role="add-balloon-Python3")
dialog.add_button(_("_Cancel"), Gtk.ResponseType.CANCEL)
dialog.add_button(_("_OK"), Gtk.ResponseType.OK)

builder = Gtk.Builder()
dir_path = os.path.dirname(os.path.realpath(__file__))
builder.add_from_file(os.path.join(dir_path, "ui_nodialog.glade"))

box = builder.get_object("box")
dialog.get_content_area().add(box)
box.show()

How can I specify on GimpUi.Dialog to expand in all dimensions automatically when the user resizes the window?

Full code are available here.