I would like to change the font size of the message text so that it is bigger. I don't want to change the size of anything else. I tried to edit the style sheet. Not only the message text changes, but also a few other elements. Can someone please advise me which style should I edit? Thanks.

I would try td.caption1 (but since it's being used by other sections, it will change some other sections too). You could duplicate td.caption1 and create a new style (for example name it td.poststyle) with a different font and apply in main_posts_cell.html.

I created a new style post in style sheet and used it in main_posts_cell.html as below:
<span class="post">{$posterText}</span>

Thanks tom322.