Fully agree with netmeg that we need a viewport based solution, not a server based one.
And since google seems to be taking away all our options to do that - from the answer dataguy indicated:
I think we're going to be stuck to just use one size unless we one of us can get through the adsense-support and have them ask their policy dept. (those are the guys handing out permissions nowadays - just had one to fix something else they borked).
Ideally the solution would be for Google to do this themselves and show an ad inside a box of whatever size we point them to dynamically. But that's far off it seems.