Hello,
All the font sizes for the top menu and body are controlled by the body tag:
template_css.css (line 763)
body {
font-family:Arial,Helvetica,sans-serif;
font-size:75%;
margin:0 auto;
}
For the image menu:
template_css.css (line 1832)
a.mainlevel-img {
color:#000000;
display:block;
font-family:Arial,Helvetica,sans-serif;
font-size:11px;
font-style:normal;
font-weight:normal;
line-height:25px;
list-style-position:outside;
list-style-type:none;
margin-left:15px;
padding-left:8px;
text-decoration:none;
}
Default Joomla menu without images
template_css.css (line 1774)
/* Default Joomla! Menu */
a.mainlevel {
color:#000000;
display:block;
font-family:Arial,Helvetica,sans-serif;
font-size:11px;
font-style:normal;
font-weight:normal;
line-height:25px;
list-style-position:outside;
list-style-type:none;
margin-left:-14px;
padding-left:8px;
text-decoration:none;}