I know that flange focal distance of K mount is rather long: 45.46 mm.
Anyway...
*ist D was 129 x 95 x 60 mm mm and it's very small camera.
*ist DS was 125 x 93 x 66 mm...
The smallest DSLR with SR is K-x:
123 x 92 x 68 mm
GXR body is 113.9 mm x 70.2 mm (WxH).
The thickness of part for module is very thin.
Camera body (excluding battery, memory card, neck strap, and connector cap): 160 g. The module weight could be very light too.
The thickness of module could be not above 55 mm. IMO.
For example, NX100 is 120 x 71 x 35 mm.
GXRII could be made a bit smaller than now. But...Anyway.
113*70*55 mm is smaller than any Pentax DSLR ever made. Am I wrong?
As for SR - I don't see problem to develop new compact version of SR especially for GXR module.
I think it's real to add simple WR to GXRII body and to K-mount module.
For tiny size Pentax made Q system.