Originally Posted by BBel
on Mac sadly...
I had looked at that and thought, hmmm even I could manage to get that working, if only...
You could just rename the epub to a zip (eg. mybook.epub to mybook.zip), open up the archive, find the stylesheet.css file, and edit it removing or changing the appropriate lines, save the file (zip file should update the changes) then rename mybook.zip back to mybook.epub and you're done.
Alternatively you could install Calibre and re-convert the epub ensuring you select the option to "disable font rescaling".