Answering my own post, for anyone who wonders...you can add a scale factor to the chat line. Paragonwiki's Demo Editing section described changing the chat colors, and I found another user guide on paragonwiki that talked about customizing the chat bubble. I combined the two sets of info...and it worked!
You can increase the size to 1.5 times normal, by adding <scale 1.5> (including the <>) just before the text. For example,
0 7 Chat 4 0 "David: <scale 1.5>Help!"