I'd also like to see such a feature. I think what Joe is suggesting is instead of using the slider bar and "testing" a few compression levels until you get it "about right", instead there would be an option in the "FTP as" and "Save photos as" dialog boxes along the lines of "save as jpeg with whatever compression level will result in XXXkb file size" where xxx is e.g. a box for a user-entered number.
That way you quickly create a known, semi-precise, resulting file-size....
Agree this could lead to not-so-acceptable quality images in certain cases, but as the user I'm willing to shoulder that responsibility...
Thanks,
Ben