I would assume almost all forms that are enabled for the panel would want to be 100% width (12). Right now that means that the default form (which is 6) has to be modified (or we enable the tablet/phone layout). If we enable the tablet/phone layout, it defaults to 6 anyway, so we have to modify the form anyway.
Perhaps you could add an option to default to full-screen/12 width either when enabled the panel, or in general.