The Finder's View » Show View Options window only allows for fonts up to 16 points, when in theory you can use any font size you want. For me, changing the size to 18 points makes it that much easier on my eyes, and leads to fewer times when I'll have to use Universal Access. To change the font size, navigate to your user's /Library/Preferences folder.
In that folder, find com.apple.Finder.plist. Before you do anything else, make a copy of this file. Now open the original for editing. You can do this in a standard text editor, or but I strongly recommend you use a Property List (plist) editor to do so. Apple provides Property List Editor free of charge with the Developer Tools (it's in /Developer » Applications » Utilities; it's also included if you have the iPhone SDK). If you don't have the Developter Tools, and you don't feel like downloading gigs of developer tools for one app, you can use PrefSetter, a free alternative.
Once you've fired up your plist editor (or text editor), find the entry named StandardViewOptions. If you're using a plist editor, there will be a little disclosure triangle next to the name; click it to reveal that entry. You should now see entries for ColumnViewOptions, IconViewOptions, and ListViewOptions. Each should have a little triangle indicating it has sub-values. In each one, there will be an option for FontSize, which has a number for a value. This is the font size (in regular font 'points') for each view. Change the number to whatever you want. You will have to change the font size for each one to have it reflected in every view.
Save your changes (see note below), then restart the Finder using Activity Monitor, Terminal, or whatever method you usually use. Other things to note:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080402181745803