The font size is dependent on three things:
(1) the zoom factor that you use in your web browser
(2) the Kunena template
(3) your Joomla site template.
The best advice (really, the only general advice) that we can give is to analyse what's happening using a tool like
Firebug for
Firefox. The next best thing is to read
Before you ask for help with template issues ...
. I know, that was written for K 1.5 but, until we've sorted out our own understanding of how K 1.6 templates work, the general principles are still fairly similar. The main difference between K 1.5 and K 1.6 templates (apart from the fact that K 1.5 templates won't work in K 1.6 - and neither will K 1.6 templates work with K 1.5) is that folder structures have changed a little.
K 1.6 is different but the overall dependence on CSS (from both a Kunena and Joomla perspective) is still the the same.