Download font resizer and save it as js file on your site:
demo.hotjoomlatemplates.com/design_now/t...now/js/fontResize.js
reference it in head section of index.php
This is code used in designnow template
<div id="font_resize"><a class="decreaseFont" href="#" title="Font Decrease"><img src="templates/hot_designnow/images/font_minus.png" border="0" alt="font minus" width="32" height="32"></a><a class="increaseFont" href="#" title="Font Increase"><img src="templates/hot_designnow/images/font_plus.png" border="0" alt="font plus" width="32" height="32"></a></div>
use code above just change image paths, you need images for plus and minus. You puit that code inside custom HTML module in HTML view