Here's a picture of the 8" X 8" case that I have been playing with.
Here is a picture of my new "test" HA server. Notice that it has four serial ports. Its in a case with a slim CD drive. The current one (collecting dust) has a dual 2.5" SATA Raid 1 setup (in a mini cage inside of the case), CF slot in the front and mini automotive style PS. While small I wouldn't hang this one on a wall. I've tested HS on this one and the Atom mITX setups.
I am currently using one with an Atom based mITX board for one automobile. It is set up with a Raid 1 array and works well. Boots up within 30 seconds in the car. I have two others set up with same case. One is a celeron based mITX 2 Gb touch screen computer and for a test relating to smallest HA server a second one with a dual core 7200, raid 1, 4Gb setup.
This one would fit on the wall next to the OPII panel that I have in FL. The mITX board is an "industrial" type board with 4 serial ports and 4 USB ports. Only one serial port is connected though. The mini PS in this case has the option of charging and running off a 12VDC battery.