There is an app that will do this - it has been discussed here before. It creates a virtual screen whatever size you want, then scales it to the native pixel dimensions.
I have to believe that looks like crap, but several people posted to that thread saying it was useful. Sorry, can't remember the name of the app.
Found it. I think there is another thread on this also... http://forums.macrumors.com/showthread.php?t=1662357
It does indeed suck, unless you have more pixels than the resolution you are targeting, that is.thanks Boyd01
will ask the guys in that post