I found this little gem today while trying to tweak Mail.app’s UI to suit my tastes.
Change the true value to false
Save the .plist file and re-launch Mail
The list will have returned to the “classic” layout (and will now show the fonts in the size you want), while keeping the rest of the app in the new layout.
The down side is that you’ll lose the multi-line preview, but you can make the thing readable this way.