I was very disappointed that I couldn't add the Size column to the list view in search results in Snow Leopard like you could do in Tiger, since Apple gave us the impression that Spotlight would include additional ways to sort results in Snow Leopard. Only Date Created and Date Modified were added as options in 10.6; the others are grayed out.

Fortunately, I found a way to force the Size column to appear, as well as Comments, Version and Label columns. You'll have to edit the com.apple.finder.plist file found in ~/Library/Preferences, using the Property List Editor app (included with Xcode) or PlistEdit Pro.

Open the Root node, then navigate down into SearchViewSettings » ListViewSettings » columns. Then choose the column you want to activate in the column list (ie. Size), and set the visible key to yes. Save the .plist and relaunch the Finder (Option-Control-click on the Finder icon in the Dock).

Note: you must have done at least one search, and selected one of the additional columns in the View Options panel for the search results (Date Created, for example), or else your .plist file may not contain the SearchViewSettings node.

Warning: Apple probably disabled these columns for a good reason, as there may be some bugs that could cause problems on your system. I didn't have any problems with the size column, but be warned.

[robg adds: I have no idea why it's taken over three years (and counting) to fix something that worked in 10.4. Until Apple gets it fixed, though, this seems to do the trick. I tested it on my MacBook Pro, and haven't had any problems as of yet. Note that you won't be able to actively enable/disable the columns in the View Options dialog, but all the columns you set to yes will show in your search results -- and once they're showing, you can sort as expected by simply clicking a column heading.]