If the steam machines are going to be made out of regular computer parts I'm going to guess it will be micro atx, or smaller, sided motherboards and cases.
...What else would they be made out of? It seems like it's just a computer with the steam OS in it.
I wonder if the SteamOS can make documents and stuff.
Almost certainly considering Linux can do that, and if it couldn't, someone could modify it to create word documents and everything.
Edit: Chances are, if you know how to build a computer, you wont be purchasing a Steam Machine since it'll just be a prebuilt computer, which you can just go out and buy the parts to and likely have it cheaper.