FitText makes font-sizes flexible. Use this plugin on your fluid or responsive layout to achieve scalable headlines that fill the width of a parent element.
From fit text someone also created slab text, but I prefer the classics. While you could use this for all of your body copy, doing so will result in a fairy losing it’s wings for every character within a <p> tag… i.e. stick to headings!
Experiment with what looks good on your site, and head over to Trent Waltons blog to see some truly inspirational headlines.