All very strange. I have tried Norton's official site and had no difficulty in altering the size of type. My immediate reaction was that you are using Internet Explorer as your Browser, whereas my default browser is Firefox. Consequently, I re-tried using IE and was still able to increase and decrease the font.
In view therefore of your original question,which has dropped to down the list to almost 20th in this section, I think it would be appropriate if you were to re-post in AB. I am useless when it comes to real computer technology but am thinking you have more than a single problem and have been unfortunate that experts on AB have missed your posting.
Sorry I can't really help but hope that you hear from Norton and get things sorted-out.
Ron.