Let's take the full-frame Canon 5D as example.
How small can this camera be and still give comparable picture quality?
What are the design constraints? Besides the sensor, the body, can the lenses be made any smaller?
The Sony Cybershot DSC-RX1 is a demonstration of a full-frame camera in a very compact body. They could made the lenses interchangeable with some effort but it would not add much bulk. If you exclude the lens, the body is less than 1.5" thick.
A camera like the RX1 saves bulk compared to a full-frame DSLR because it can mount the lens much closer to the sensor since there is no mirror in the optical path and no pentaprism to direct an optical image to the viewfinder. So, if all you care about is image quality and neither ergonomics nor an optical viewfinder matters to you, they can make it much smaller than a DSLR, as small as an RX1 at least!
Lenses for a camera like the RX1 are only marginally smaller because the full-frame image circle must still be covered. For this reason, the size advantage is most significant for normal or moderate wide-angle lenses, my guess is between 28 and 75mm. Beyond that, the lens will dominate the camera size.
Now you did not specify a time-frame but technology evolves and while larger sensor always have an advantage based on the laws of physics, smaller sensors keep improving. So today we have APS-C sensors that match the performance of the previous generation of full-frame ones. In a few yours, APS-C sensors can match today's top full-frame models but - of course - by then newer full-frame models will be even better. In other words, within 3 years, I would expect a camera like the Fuji X-E1 to match today's best full-frame cameras.
In the body, you need to be able to house the sensor, power supply, controls and memory. then you need to able to mount the lens.
Finally, the design has to be usable. Hey presto, that is your size constraint; squeeze all that into a body and you end something shaped like an SLR.
I think manufacturers could start to move away from a basic rectangle and design the body to improve ergonomics, especially with shaping it to give the left hand a better interaction with the body and improve holding.
As the for lens, it's physics. The f number is ratio of the length and aperture opening, to get a wide aperture then the lens must be a given size.
I'm always amazed at how big enthusiast and pro DSLRs are. I have an ancient Nikon F, it was the pro photojournalist camera of the 60s, 70s, and even into the 80s. Its tiny compared to my Canon 50D, and the 50D is tiny compared to the FF pro bodies. Lots of the pro bodies have two grips, for portrait and landscape, they are two to three times the size of my Nikon F.
But the F has no batteries in the body, no memory, CPU, viewscreen. Its not clear that customers would accept a DSLR without a viewscreen. The CPU, memory, and related components are very small today, and getting smaller, so in theory they could get down to something that would fit in the Nikon F's FTn finder.
IMHO, I dont' see it. The mirrorless m4/3 are clearly the future for high quality, small cameras. I don't see small and DSLR as words that go together.
If you just want the same image quality, you could make a much smaller camera.
If you also want it to be able to take more than one image per minute you need to fit fast image processors and a large memory buffer in it. If you want to take a lot of images without changing battery all the time, you need to fit a powerful battery in it. If you want a camera that you can hold in your hands with easily accessible controls, you have to form the camera body a certain way to make a hand grip and fit the controls.