I'm always striving for / dreaming of wide angle and shallow DOF, so for me, the larger the sensor the better - or a smaller sensor with super fast lens. Unfortunately both large sensor nor super fast lens tend to make for large cameras/lenses, and that's contrary to my other wish: a camera that basically disappears when I'm not using it.
I think the rumored
sony 2/3" type curved sensor with a lens of ~21mm f/3 equivalent might meet my minimum criteria for DOF control, and it's obviously very small (smartphone), so if the lens is right, there's no real need for me to have a larger sensor than that. Still, if I were to design a camera/lens combo myself, it'd probably be APS-C at least.