Are you planning on putting the monitor in the new encolsure as well? If so, the smallest thing you'll be able to pull off is something very near the size it is now. Just measure it for the dimensions.
If you're talking about putting the logic board inside something different, without using the monitor... That can be a big pain. The Analog board that supplies power to the system also contains the video circuitry. There are ways to do it, though. I suggest checking with your library for "Build Your Own Macintosh And Save A Bundle". The author puts the motherboard from an SE into a PC case and discusses options for the video.
All in all, it's more trouble than it's worth.