Yes, Mick is correct, the Search_x=true in your links is the cause of the problem, if you replace Search_x=true with Search=true, the links will work correctly, for example try:
Products_Results.php?Search=true&CategoryID=4
to change the height of the top nav div, change the height property of the #topNavigation selector in the common.css file.
You will also need to change the search form elements by modifying the input[type="text"] selector and the #Search selector.