Currently the only phone running ICS "officially" that's physically smaller than the Galaxy S2 is the Nexus S. You could switch to a Galaxy S and install a custom ROM though, and that goes for almost every other phone there with enough hardware power to run ICS.
The main problem you will be facing is that among the phones that are physically smaller than the S2, none of them have comparable hardware: you'll be faced with slower CPUs and inferior GPUs. The only other phones that are as powerful as the S2 are about the same size or bigger (Galaxy Nexus, Rezound, Sensation, Razr, etc).
So mainly your choice would be to get a smaller, slower phone and install a custom ICS ROM, or just stick with the S2, and install an ICS ROM if you can't wait for the official ICS update.
