You're going to love me forever:
<key>9002</key> <dict> <key>- loc hint -</key> <string>List Contents (Small)</string> <key>font</key> <string>HelveticaNeue</string> <key>size</key> <integer>12</integer> </dict>
It's that interger "12" that's determines the size. Adjust it until you're happy—provided that it's an interger.
I actually go with "HelveticaNeue" with a size of 11. I like to see lots of tabular information at once, horizontally, without changing my resolution… within reason, e.g. Fiona Apple album titles still have ellipses.
An unfortunate aspect to this is that the row height isn't altered unless the font size specified is greater than 12—the font takes the row height along for the ride when it's bigger, but the row doesn't come along for the ride when the font gets smaller. It's likely you could change the minimum row height to be lower, in some other plist—search for something like "list contents (small)" or "list (small)" or whatever—but I don't know where that would be found, or if it could be found at all.
Nonetheless, this should address your query.
Further to this, you can adjust the font size of the items in the lefthand sidebar, using strings 1304, 1306, 1307, and 1308. However, that row-height issue comes up there too, and I suspect it couldn't be rectified on that side, since there are little icons in each row there—bummer-sauce. (Although it's really more the row-padding that is annoying). Actually, that makes me notice that the text boxes on the list view don't scale with the font size either—that's not to say that a smaller font would be terribly disproportionate to the checkboxes though.