I have tried the Font-size method as stated by Granny Luddite and it works on some books, and does change all the font sizes to 1em.
What works slightly better for me is to convert the epub to mobi and back again to epub. Most of the larger/smaller font-sizes are retained and the line hieght works although I have not reconverted enough books to say for sure that it will work in all cases.
The books I have done all worked fine before reconverting on the Sony readers and the calibre viewer and still do. But not on the Kobo mini.