The image on top (Click on it to view the full size) shows the first real 12-35mm and 35-100mm samples (Source: DC.watch). Both X lenses will come within the first half of 2012. All we know now is that they are fast. Our reader “Monodrift” (Thanks!) made a 14-45mm versus 12-35mm size comparison. You can click on it to see the full size image:
As you see the 12-35mm is not a lot bigger. But I do hope it is a lot faster anyway!