It's about the size of a CD case.
I’ve been using older PCs as my home router(s) for a while now, and for the most part they work great. For about a year or so I’ve been using an older P4 Shuttle w/ 3x NICs as my pfSense box. I been quite happy with the setup, but because of reliability issues and power usage I decided to upgrade to an Alix board. I was concerned about how the 500MHz AMD Geode CPU would perform. I’ve used the OLPC laptops and they’re crazy slow to use as a PC. The embedded version of pfSense is built on top of nanobsd, and I can’t believe how fast it performs on this board. It’s truly amazing. Full specs are here.
I ordered the kit from Netgate to keep it simple. My only complaint is the crappy compact flash card it came with. The Sandisk card has the removable bit set and the 2GB pfSense image does not fit. DD reports one record left when it fills up! I had to switch to the 1GB image which kind-of sucks; I may get a better CF card at some point. It’s not really that big of a deal though. You can checkout the kit here. I highly recommend these boards, and I plan on adding a wireless card to this unit at some point.