On the conversation buttons for example, you need to use a new font style that is associated with the button style. In ProxyActor, I started a simple button style that uses Zapf20 like this:
<ButtonStyle Name="text_temp" NormalTextColor="#F0D080" NormalTextStyle="/Fonts.FontZapf20" RStyleDefault="/ButtonStyles.BigRoundButton.biground_normal" />
and then replaced the style on the conversation button with it:
<Button AutoResize="true" Margin="15,10,15,10" MaximumSize="300,16384" MinimumSize="75,28" Name="ReplyTemplate" ScrollExtent="75,28" Size="75,28" Style="text_temp" TextMaxLines="255" Visible="false">:78406b312303306e:[DEVL]Tell me more about the person who stole your sword.</Button>
This gave the results shown below. You would need to fill out the other parts of the button style (pressed, mouseover, etc.) with the larger font to make it viable. Look in eq2ui_fonts.xml for some other font choices. I would also be fairly cautionary about changes like this -- you'll want to test it out with some longer responses to make sure they don't get cut off. I would also check to see how it works with many choice buttons -- talk to one of the guild harvester NPCs for example.