It will help us to help you if you post a link to your site
The bad news is that the font size has been set on lots of the rules in the themes style.css file so changing them will entail going through the style sheet and making changes (or removing them completely and going with a default setting except where necessary, for headings etc).
There appears to be little consistency in setting the the font size with some rules using pixels and others using ems. Personally I tend to use ems for all my font-size settings.
Be aware that changing the font-size may entail you changing the line-height as well to avoid the line spacing looking cramped.
Grand, so could you give me a guide on where to start?
Thanks
