Yup, i just added "font-size: 8pt;" into the section "div.Message" in MessagingWidgetsChatConversation.css and it works like a charm.

Obviously you would have to do the same with MessagingWidgetsSingleSMS.css and MessagingWidgetsSMSConversation.css if you want to change those too.

Of course remember this is a hack, it wont hurt anything but it may be wiped or not work correctly in a future firmware update. I'm now even more in love with my N900 knowing I can tweak these sorts of things as CSS and HTML are the things I spend hours tweaking as it is.