Upload & Sell: On
I opened an image with a very defined feature in Photoshop on my desktop computer, at 100% pixels. It measured 122mm. I reduced it to 50% pixels - it measured 61 mm. I reduced it to 33% pixels; it measured about 40mm. So far, just as expected.
Then I opened the same picture, still on the CF card, and magnified it as high as it would enlarge on the rear LCD of the camera. It measured 48mm. So the maximum magnification is 48/122 or about 39.3%. An interesting fact, which I've never seen published anywhere.
The image was actually shot on a 5DII, which I don't have anymore. But my test was computer screen vs 5DIII screen, so I'm not sure that matters.
It would be interesting to try this on other Canon bodies, and see if they present the same maximum magnification.