If you look more closely, you will see that the description is actually controlled by this:
Code:
<SPAN class=>
{if $product.fulldescr ne ""}{$product.fulldescr}{else}{$product.descr}{/if}
</SPAN>
As you can also see, the class is empty, so it is defaulting to the font size in the body tag. Add your own class or call any other class with that span tag.