I'm confuseed .. the viewer doesn't do anything at all to fonts it shows you the remote desktop basically using your own font size on the local screen.
I control over 500 computers remotely and i can't fathom what you are trying to do with this question ? If you can't read the text you need to lower your resolution on your local (controlling) computer.
>> Are you referring to the text (Telnet) connection mode? <<
No, I don't. The fontsize I would like to change is the displayed fontsize of the Viewer 3.0 INTERFACE (the "phonebook"). I have to use 120dpi style on my 19"schreens because the default fontsizes of many interfaces are to small for me. But the Radmin "Phonebook" default font is to large now, I would like to change that to an 8 point font or so.