Michael-
Font sizes can be set in many places so there is no single place to look.  It can be defined in the default portal CSS, the applied skin, the applied module container, something set in the header of the page or module or even hard-coded in the module itself.  In practice you'll usually find that your CSS is a result of defined in many places.  A common example: a module's text hyperlink CSS may be defined in the applied skin file while the module title's "header" text is defined by the applied container.

Best Way to Track Down CSS
Rather try to manually hunt down element attributes, you should use a tool that reports all applied CSS on an element and where each CSS attribute is defined.  There are a number of helpful plugins for FireFox that can do this.  For IE I like to use debugBar which I've blogged about (with screenshots) here:
 
   http://www.eguanasolutions.com/DNN_Blog/EntryId/19.aspx

With the plugin active (you can turn it on/off), you just need to highlight a page element to get all the CSS info including which files contain the relevant CSS definitions.  This is my standard method of tracking down CSS info when I want to alter a style -- I can determine what CSS is being applied and where it is coming from in under 30 seconds.

IE8 and Google Chrome have basic CSS reporting built in.  Microsoft also has a very-similar-to-debugBar addon called the "IE Dev Bar (Beta 3)".  Much of this dev bar was incoporated into IE8.

 
Good luck!
-mamlin

Thank you both for the help.  The styles i was looking for were in the module itself.