How can I sort a Finder search by size on OS 10.6.8?
This should do it:
Add columns to Spotlight search results
Quote:

Using Property List Editor
If you are using Property List Editor, start by going to your user folder and then navigate to Library/Preferences. In the Preferences folder find the file named “com.apple.finder.plist.” Double-clicking on this file should open it up in Property List Editor. If not, just right-click on it and choose “Open With - Property List Editor.”
Once the file is open, scroll down the list to “SearchViewSettings.” Click the disclosure triangle next to it, and then do the same for “ListViewSettings” and then “columns.” Now you should see a list of all the available columns. Click the triangle next to the column you want to add, i.e. size, and then check the box next to “visible.”



Once you are done, save the file by pressing Command-S. Finally, you will need to restart the Finder for the changes to take effect. The easiest way to do this is top hold down the Control and Option keys, click on the Finder icon in the Dock and choose “Relaunch.”
Now just press Command-F to bring up a search window and you should have your extra columns enabled.