How to change defualt text size

Have you got any knowledge of Child Themes? If not have a quick read here:

http://codex.wordpress.org/Child_Themes

when you have finished reading it, you should download and activate this plugin to create the child theme for you:

http://wordpress.org/extend/plugins/one-click-child-theme/

Then it is a matter of writing a short CSS code and it should work but first do the things I have suggested above and then post back with a link to your site.

G/L