7
If i am understanding correctly you want to make the text size of one link in big font?
try
<span style="font-size:x-large"><a href="http://yourlink">YOUR BIGG TEXT</a></span>
font size is pretty much deprecated now its always best practice to define presentation in css, the style="font-size:x-large" in the above code is the css rule to make the text within the span tag extra large size.
Another way of doing it is to give your <a> anchor tag an id (or class if you want to use it more than once on the same page) and set a css rule fot it in your theme css, example
<a id="mybigtext" href="http://www.link.com">Big text here</a>
and in the css file of your theme you can type
#mybigtext {font-size:x-large}
or wrap link in <span id="mybigtext"></span>
