I've been using Lexia DaMa as the alternative to Caecilia. I typically set font size somewhere in the range from 11 to 13 pt depending on which font I am using. I did not want to hack my 950's firmware. I also use the calibre Extra CSS method which works for non-DRM books.
Here is a linked post. See the attached PDF at the bottom of the post. Jswinden has written a very detailed (but easy to follow) guide to writing the extra CSS and modifying your EPUB either with calibre or an editor such as Sigil. Sometimes it's not as easy as just modifying the body tag and converting in calibre.