No kidding they recieve the pixel size of the file, BUT ACTUAL PHYSICAL PIXEL SIZES VARY!
Look at the pixel size on a 1600x1200 monitor vs. the pixel size on your cell phone or a graphing calculator. Guess what? They're different!
Most comptuer monitors have a pixel size of 0.26 mm to 0.35 mm.
Large presentatoin monitors sometimes use .48mm pixels.
But again, pixel size is dependent on the resolution of the persons display. Switch your monitor to 640x480 and try to measure the size of a pixel on your monitor. Now switch to 1280x1024 and see if you can still measure it.