I have found a way to change font sizing in the itunes print options. You have to be comfortable editing the iTunes XML files. I tested this on iTunes 11.0.1 on OS 10.8.2.
Start by quitting iTunes. Then, right click iTunes.app and 'Show contents'. The folder is 'iTunes.app/Contents/Resources/English.lproj/PrintingTemplates' (I assume this is valid for the other languages, but I'm not sure). You'll see the xml files inside corresponding to the print options given.
Open the one you want with a text editor (I was interested in 10.Insert.SingleCover.xml). I strongly recommend making a copy of the file elsewhere, or you can restore from Time Machine if something goes wrong I suppose.
Inside there will be a line
<element type="playlist">
(line 65 in my file)
Below this there will be a line with something like
<textSize>12</textSize>
(line 71 in my file)
Changing that number will change the font size for the playlist text (track #, song name, playtime). You may also need to change the next line down, <smallestSize> to avoid any problems but I'm not sure.
For the Playlist/artist combination (the top), look for the string <textFormat index="0">%COMBOPLAYLISTANDARTIST</textFormat>. Two lines above it there is another textSize and that controls the title font size.
You have to save the file and reload iTunes for it to take effect. You will (probably) need to authenticate as an administrator for the file to save.
Good luck!
edit: fixed html