The Pentax K-x is 4.8 x 3.6 x 2.7 in. and weighs 27.9 oz. It's quite small and is difficult to handle if you have big hands. Pentax has never mentioned it to be a compact dSLR.
Canon refers to its smallest entry-level dSLR's as compact dSLR's. Their smallest is the Rebel XSi which measures 5.1 x 3.8 x 2.4 inches and weighs 16.8 oz. It's a regular bulky monstrosity you can never put in your pocket. It's just their smallest not really compact.
The Olympus E-420 is the smallest dSLR ever. The Evolt E-420 measures 5.1 x 3.6 x 2.1 inches and weighs 13.4 oz. It has a four-thirds mount and a digital sensor that's smaller than APS-C. The E-240 however comes complete with a reflex mirror and a pentamirror (or pentaprism) that are the defining parts of a true dSLR. It's really small but it's not compact.
Make the reflex mirror permanent to take away the pentaprism/pentamirror and you'll have a translucent mirror camera or Single Lens Translucent (SLT) like the Sony Alpha 33. It's 4.9 x 3.6 x 3.3 in. and weighs 25.6 oz. It uses an APS-C sensor. Even without a pentaprsim or pentamirror, it's still visibly bulky and can not classify as a dSLR due to the lack of a pentaprism or pentamirror. The translucent mirror is there for rapid autofocus functions. The camera is obviously flattened but compact? Maybe.
To make a dSLR more compact, you have to remove the reflex mirror. What you will get is a hybrid or mirrorless camera like the Samsung NX10. This one is 4.8 x 3.4 x 1.6 inches and weighs 12.5 oz. It uses an APS-C sensor. Is it a compact dSLR? No. The reflex mirror is gone hence it's no longer an SLR. You can no longer call it a dSLR. Compact? Close.
If you make the dSLR smaller by taking away the reflex mirror, the pentaprism, and then make the lens permanent, you'll end up with something like the Pentax X5 which is a bridge camera. This one's at 4.7 x 3.4 x 4.2 inches and 17.9 oz but other bridge cameras are much smaller. The 1/2.33" sensor is too small for dSLR standards. It may resemble a dSLR but it lacks the defining parts of a dSLR. It can't be called a dSLR. Compact dSLR? Never. Compact, yes.
Take away everything then and replace the sensor and lens with small ones. What you'll have is a point-and-shoot camera. A real compact camera.
Now you know!