Going through some pages of Customize N900 Conversations I managed to increase the font-size of IM conversations. Procedure follows simply as:

Go under /usr/share/rtcom-messaging-ui/html/
and edit MessagingWidgetsChatConversation.css

Locate div.Message { }
and add or modify font-size: 32 px

32 px is my current setting. Not sure how to alter other font size system yet unsure if any exist. This is the most required one for me and I have a solution.

I am looking more generic ways to increase font size in conversations and making it full screen to gain little more space. Jump in if you are interested in -> Two questions about conversations app