inside the exchange part of the element I'm now using
dialog = this.getDialog();
alert(dialog.getContentElement('info', 'grootte').getInputElement().$.id);
and as a result it gives "cke_117_select". (This is selectbox)
alert(dialog.getContentElement('info', 'txtUrl').getInputElement().$.id);
gives 'cke_107_textInput'.
I think this is what you (or other visitors to this page) are looking for.