Why not just get a laptop? Small form factor, and it's portable.
I generally don't like small form factor PCs. They're expensive. The really small ones are especially expensive, for the hardware you get, and cannot be upgraded. Then there's things like the Shuttle case, which is like a large shoebox, and can use standard desktop PC parts, but they too are pretty expensive and need custom motherboards due to their small size. When you finish fully stocking one with processor, RAM and a video card, you haven't saved anything over a standard desktop case.
So again, I recommend a laptop.