I ran an experiment on Google Chrome (code below) to determine how line length varied with font size. it looked pretty linear.
Below is my guess for the curve. You can see it on Wolfram Alpha:
(line length) = 25.407 * (font size) + 6.3799
Here is the jquery/html code i used to generate the results:
the initial inspiration for fitting the curve of font size to line length came from a post by ingeva on daniweb.