No! Don’t change the body font size! That’s really bad practice (and yes – I know the default theme does – what can I say). Find the problem first and then change the font size on that specific block/element. Right now, you have a body font of 20% set in IE which frankly, is ludicrous! I can guarantee that such a setting will come back and bite you.
Despite what was said above, start by sorting out the validation errors on your site. It won’t cure every problem but it does provide the best possible foundation for cross-browser performance. For example, your meta-keywords are totally screwed up…