Adjusting font size - fixed width or fluid?

Adjusting font size - fixed width or fluid?

So I'm designing a portfolio page which will contain mostly header text
with a few paragraphs and images thrown in for good measure!
My headings and one lined text will expand to fill the entire width of
it's container which will be the wrapper.
<div id="wrapper">
<h1>I'm a heading</h1>
<p>I aim to fill just one line regardless of letter count</p>
</div>
Now, I have been searching the net for a solution which will allow me to
achieve the above. I have found both bigText.js which doesn't work no
matter what I do and fitText.js which doesn't automatically size text to
it's parent container without editing the .js to find a font size that
suits - even then it's not pixel perfect.
My question therefore is, With a wrapper that I wish to keep just 35% of
the screen (centred), am I best to keep as a percentage (fluid) or use a
fixed width? and with either option, what's the correct way to adjust the
font-size to fill the wrapper width correctly?
Thanks in advance,
Simon