I am sure that many people who are using Joomla is looking to change the font size at some point of time. However, it is not possible at the time of writing to do so (for Joomla 1.5.9) without any workarounds.

The first workaround is by changing the font properties in the CSS style sheets of your template. You can access the style sheet by going to Extension >> Template Manager and selecting the style sheet. Locate the font-size attribute and increase that size respectively. Fortunately, there is another way =)

The other way is to upgrade the default Joomla Editor to JCE Editor =) This is my preferred way because JCE Editor not only allows you to change the font size, this powerful Editor also has tons of other neat features as well. The best part is that this amazing plugin is free!