I think I finally understand what you're saying.
You're looking for the real world width/height that the pixel represents. The pixel itself doesn't get larger (a pixel is a pixel) but the area in the real world that pixel represents at 5mm vs. 5m is different. At 5mm from the sensor, a pixel
represents a very small width/height in the real world, but at a distance of 5m from the sensor, a pixel represents a much larger width/height in the real world.
So, after going back and forth on this, I'm not sure I have an easy answer for you. :)
There is nothing built into the Kinect SDK that would give you what you're looking for. About the only suggestion I would have is to point your Kinect at a flat surface at a known distance, and measure the width/height of what it can see at that distance
(i.e. watch yourself on the cam put a tape measure at one end, and run it to the other end of what you see on the display and get the distance). After dividing it out at that distance, you should be able to determine a relationship as to what width/height
in the real world a pixel on the camera would represent. From there, you should be able to extrapolate what that width/height would be at varying depths.
So, all that said, I'm not entirely sure how accurate this would be due to the curvature of the lenses on the extremes and what that would do to the values on the extremes. It's likely that the edges would actually be larger widths/heights than the
center areas due to the distortion.
Hope that makes some kind of sense.....