Ok so I'm not sure if I'm the only clueless one on this, but I figure since it took me awhile to find it, it might be useful to others out there.
When using the new Burn folders in Tiger, the system creates aliases to the files you want to burn. It's a good idea since it now (finally) allows a Finder burn without the dreaded disk image creation. But since the Burn folder consists of nothing but aliases, the size report you get for it (or the items it contains) doesn't reflect the original items. This makes it impossible to know the actual size of the backup you're creating!
Well, it turns out that if you simply click the Burn button -- without a blank disk inserted -- you get a dialog telling you the size of the disk needed for the operation. I still think they should put this in the Burn folder's window (a la Toast), but at least I can now actually use this feature.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050524095141126