Hi,
Having not found the answer on the forums, I did my own search. You need to modify the file Viewtype_Multiplex.xml which is located in addons\skin.neon\720p\
You can open it with any text editor (I used notepad++), look for "plot" (without quotes!). There will be 4 instances of plot found in that file:
Line 113: <label>$INFO[Container(588).ListItem.PlotOutline]</label>
Line 318: <label>$INFO[ListItem.Plot]</label>
Line 326: <visible>!Skin.HasSetting(extrathumbs) + IsEmpty(ListItem.Plot)</visible>
Line 331: <label>$INFO[ListItem.PlotOutline]</label>
there's a <font> tag closeby to these items, I found that changing it to
<font>Font_Neon_34</font> worked nice.
You can change the font size of the other components too : e.g. "direct by" label and the name of the director. I changed these other font sizes too to and changed their distance a bit to make it all easier on the eyes.
Remember to restart XBMC to see the changes.
If you like I can upload my modified file so you can replace yours (after first backing it up of course).