I’m using 3.0.
Go to wp-admin and enter your name and password
Go to the page you want to edit
On the left side, go to appearances
Go to Editor — on the right side at the bottom is your style sheet
If you want the code to use, it is <span style=font size:””>
within the two quotation marks you put the size number you want
See: http://www.tizag.com/htmlT/font.php
