Kmail Font Size Issue

Hello. I'm trying to set a minimum font size in Kmail. I've set the size using the settings -> configure kmail -> appearance dialog for every type of text, but on some messages (HTML format, maybe?) the message shows up as very small text and I can't find a way to make it larger (e.g., at least 12 point). This is the case in both the message preview pane and also when I fully open the message in its own window.

I've tried:
1. Changing the font size using Settings -> Configure Kmail -> Appearance dialog for every type of text (e.g. message body, etc, etc)
2. Right-clicking on the message and choosing to use the "Fixed Font".
3. With the message in its own window, using the toolbar option to change the font size to a larger one (e.g., from 12 pt to 16 pt). In this case, the font size for the message header changes, but not the text of the message.

Is there a configuration file I can use to over-ride the font size for certain types of messages, like HTML? Or any other solution someone can suggest?

I'm on Kbuntu 8.04 using KDE 3.59, Kmail 1.9.9.

Thanks for your help.
Dave