Late reply to the question; Defining your font size to a fixed size (px) or (pt) would mean that your site visitor can't increase the size of your Web page text. You might say "great", but this doesn't allow someone with visual disabilities to adjust browser settings to increase font size. If you're designing a website for the general public or for the government, this is a big "no-no".
To be on the safe side use relative measurements (em) or (%) that allow for font adjustment in a browser.