As noted here in previous hints, you can't find Mail messages in a normal Finder search -- you either have to search in Mail, or use the Spotlight window. However, I wanted to run a search in the Finder, so I could easily see the size of all my saved Mail messages, regardless of which folder they lived in.
The answer is simple, and is alluded to in the previous hints (which show how to modify some files to make Mail messages always show up). However, I though it worth posting directly, in case someone is looking for a simpler solution that doesn't require modifying files.
In the Finder, navigate to your user's Library » Mail folder then press Command-F. Make sure the Folder "Mail" button is highlighted in the search bar, and then click the Kind pop-up menu. Select Other, and then scroll down the monstrous list that appears and find Raw Query. For the query string, enter kMDItemContentType = "com.apple.mail.emlx", and that's it. If you want to save the query for future use, click the Save button.
Once the query has run, you can press Command-A then Command-I to open the Get Info window and view the size of all the messages. I'm sure there are other ways to do this, but it was the easiest one I could come up with that accounted for all the various subfolders. You can easily build a Smart Mailbox in Mail to show all your stored email (I use Date Received - is after the date - 01/01/93), but I don't see any way to then display the total size of all the collected messages.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070725152123886