Once you solve the other issues
font size is usually a piece of cake ( except when: see @Toxaris post).
In the Style sheet:
find the class used in the <body> tag. change the
font-size: value
(test results by looking at many different types (in book usage) pages)
If only a few areas have issues, spot fix their classes font-size. If not, you are going to have to work backwards through the nested 'box model' to find the culprit.
IMHO 300+ line CSS for a
simple book = a GIGO issue