There are two options to increase the font size on your phone:
The first is to simply install a new font that is larger than the default font. Font Changer and Type Fresh are two Apps that allow you to change Fonts on your phone (they do require Root access though).
The second option is to use dedicate Apps to change the font size, although they will normally only work for SMS and some Google Apps.
To enlarge the SMS font, go to the Android Market and download an App called ChompSMS. Among other things, the App allows you to increase the font size of your SMS messages.
For enlarge SMS and the standard Google Apps fonts, then download an App called SMS/Talk Text Enlarger.