I suspect the problem is your "universal remote" doesn't have all the functions of the original remote including the size or aspect ratio button. If you can't get a replacement original remote then get a Logitech Harmony remote. These are programmed online with their huge database of IR codes. You will have all the functions of the original remote and sometime additional codes like direct input buttons.
You cannot change a TV's screen size, that's physically impossible.
I assume you meant aspect ratio, or you have a border around an image? If so, that'll be an issue with the device providing the signal more often than not (cable box, DVD player etc) and should be addressed within it's respective settings menu. Potentially within the TV settings, if applicable (most TVs will have at least rudimentary settings options)