Font size is controlled by CSS AND the users browser. So you only have some degree of control over it. You can also set font size in various places depending on how the site is coded and what you want to do. Firebug is the best way to look at CSS and figure out what you need to change to achieve the appearance you want. At the most basic level, if you want to make all the fonts in the entire site bigger, you can change the font size percentage in the following CSS:
background: none repeat scroll 0 0 #777777;
So you could try changing the 100.1% to 101% and see how it looks. But beware that changing something like that may mess up other things.
If you want more specific changes (i.e. only specific sections or headers or whatever) you'll have to look at more specific CSS that applies to more specific elements.
See this page for more info on stylingfont size.