If you want to edit the files I would recommend you 2 things:
Here's the first thing you need to know: www.rainlendar.net/cms/index.php?option=com_easyfaq&Itemid=26#faq6
then, edit the /xml/elements.xml and duplicate the font elements font.normal and font.bold. Rename both to e.g., font.normal.large and font.bold.large (keep all the 4 to avoid break anything else)
Finally, edit the /xml/todolist.xml and /xml/eventlist.xml and change any mention of font.normal and font.bold to your new font elements.