As people have already pointed out below, this is a case of either/or: either you get the pocket size, or you get the big sensor.
There is a third option, however: you could get a sensor that has an extreme setup of micro lenses. If you want your lens to stay small (because it needs to be pocketable), you will need to compensate for the light falling into sensor "pixels" at a serious angle. Sensors don't like that at all, so you'll have to put microlenses on top of each "pixel" to bend the light so it falls onto that "pixel" at a much straighter angle.
The problem with a sensor of that size, is that you won't be able to reuse any of the common lenses. I was thinking of suggesting you to design a camera with a mount (e.g. LTM), but those lenses aren't guaranteed to cover your sensor. Adding a Hasselblad mount would be crazy, because the lenses will make the camera unpocketable. Conclusion: you'll have to design your own lens, or let another company design it for you.
Cosina is a Japanese company that designs lots of cameras for others. Maybe lenses too, I'm not certain.
Estimated cost for such a camera if it would ever reach the market: $5000 or more. Except if you've got really deep pockets and you can order large quantities. If you just want a few prototypes for yourself to play with, I'm quite certain none of the companies mentioned in this thread would be interested in working with you. In that case, you'd have to do everything yourself. Have fun designing the lens, the shutter mechanism, the sensor and ASICs, ... ;-)
If you think I'm trying to dissuade you by making it sound impossible, you're partly right. It will be an extremely difficult job and I want you to realise that. But please, do go ahead. Most geniuses were called lunatics before they succeeded
Peter.