I think there are two questions here, with two possible partial answers:
1 - how small could they make a DSLR? I have a film EOS that's much smaller than any digital, so could they shrink one down further? I think the film camera size comes down to only needing space for a thin strip of film before the back of the camera. Now we have a sensor with filter assembly on top of it, and typically a LCD display behind that. Let's say the rear LCD was ditched, and they just relied on the viewfinder for everything (possibly X100 style electronic-hybrid?). How small could that get? Would it be interesting to people? With a small prime on I think it wouldn't be that dissimilar in size to current bigger mirrorless models.
2 - if they go mirrorless, where in the range would they target? The m4/3 and APS-C sensor size is pretty much covered by existing players for example. Could Canon bring something new to that space and not come out with a "me too" product? The tiny sensor like Q I find hard to take off, as you're competing directly with compacts there, and to me I can't see why I would want one over a compact unless the price is slashed right down, but then what's the point? The rumoured small-ish sensor Nikon is more interesting, as you can get smaller lenses than current APS-C models (assuming you're not after strict DoF equivalence) without compromising too much on image quality. That would be a more logical positioning keeping both compacts and SLRs out of the way of self competition. As a wild card, could they go high end only? Go full frame, and make a Leica for the 21st century. I don't think they would have any trouble undercutting Leica pricing anyway...