Look at http://www.phpbb.com/styles/demo.php
then look at the source code; at one place it shows
/* General text */
.gen { font-size : 12px; }
.genmed { font-size : 11px; }
.gensmall { font-size : 10px; }
/* The content of the posts (body of text) */
.postbody { font-size : 12px; line-height: 18px}
a.postlink:link { text-decoration: none; color : #006699 }
a.postlink:visited { text-decoration: none; color : #5493B4; }
a.postlink:hover { text-decoration: underline; color : #DD6900}
so that gives you some idea. The font size never seems to be over 12px and a lot of the general text is 10px.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.