I.e. fit into messenger bag? Maybe even Macbook Air would fit. You actually have some selection of smallish notebooks around ~10 inch, or x86 based tablets. Old EeePC line might be a good fit, if you don't need much power. You might even opt for iPad and keyboard-cover.
Personally, I have bought myself an underpowered arm-based 10' lenovo chromebook. Has decent enough keyboard for touch-typing, and I can run most of my dev-setup with the linux support.
Do you need to go smaller? You are then entering a niche teritory, where you might want to try Gemini PDA (a Psion 5MX clone running android or linux with surprisingly good keyboard that can fit your pocket), but every time I considered it, I figured it is too expensive for what it would give me.