Macbook to Dynex TV

I was looking into getting a second tv to also use as an external monitor with my macbook i hooked my computer using the Dynex video adapter to my friends Dynex TV ( http://www.bestbuy.com/site/olspage.jsp?skuId=9135385&type=product&id=1218054461 986) but the the screen size on the tv was only like 15 or 16in rather than being the full size of the tv. anyone know how to make it the full size of the tv other wise this isnt the most viable option for me. any help would be much appreciated

Mark