Assuming the desired image size is supported by your camera, an Autostart script with 'set_resolution' followed by the appropriate number should work.
Good point - I made the assumption the op was looking for custom resolutions. He might just be looking to change standard resolutions on the fly. Although if all he needs is one of the built-in resolutions (i.e. L,M1,M2,M3,S,RAW,Postcard,LW,SW) it would probably be easier to just select that from the Canon menus.
