• look at the XHTML source code
  
• find the first subject line
  
• now go up in the documents hierarchy and note all tags and css-classes
  • < a class="thread"> (for a "normal subject line", or sticky if fixed, or new)
    
  • < li>
    
  • ... up until < body> is reached
• until < body> is reached
  
• open style.css
  
• now go down according the hierarchy you have noted in XHTML
  • starting with body { font-size:100.1%; }
    
  • until a.thread, a.thread-sticky, or a.threadnew is reached

That's the wonderful thing with a liquid design: all font sizes are relative to the one given in the body.

Therefore:
body 100.1%: the default font size of the user's browser is used)
content p and content ul: paragraphs and list are set to 0.82em (=82% of body's)
table class="normaltab": almost there
table.normaltab td.a: even closer... (0.82em)
table.normaltab ul.thread: here we are (font-size:1em !important)

That's an example for the main page; if you want to change the font size of the subject line in an open post, just use the same method (HTML up, CSS down).
Hint: h1 class="postingheadline"...

@Alex: Oops, missing in style.css, therefore the browser's default definition for

<h1>

(within the "content-class") is applied...
BTW, this is semantically not correct; should be < h2>, because only one < h1> should be used within a page ("There can be only one!").