I can think of two options:
Firstly, return to the last restore point. This may not work as I'm not sure if the font sizes are held in the System Volume Information directories.
Secondly, if you still have the MSOCache directory on your hard drive you should be able to perform a repair installation.
MSOCache is a hidden system directory containing a backup of the Outlook installation. Open Control Panel, then "Programs & Features" - this is named differently on different versions of Windows and even differently named on the different display modes in Windows 7 so it could be called something else on your PC but it is reasonably intuitive.
You will get a list of installed software. Right-click on Outlook and choose "Repair". This will restore a large number of settings using the MSOCache records. You may find you have to re-enter your account details but the OST file should still be recognised by the repair.
As a precaution, I would back up your PST file first.
However, if you have already recovered the font sizes on the desktop and elsewhere, it may well be something simpler than either of these. If I think of something simpler, I'll post it!