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If you go by numbers alone, then you will always come to the wrong conclusions about how large a print can be before losing sharpness, clarity, etc. There are many more factors involved in a printing outcome than just MP and dpi/ppi. The fact is, I'm staring at a 16" X 20" of the white flower pic shown above, and it looks exactly like the image you see here on your screen, if not better. Indeed, I've seen 20" X 30" prints from the 1D that, at proper viewing distance, look excellent.
Yes, they might look good...but compared to what? Take the same photo with a 5d3 and print it out and see the difference. I have photos hanging on my walls shot 25 years ago and they look good, but I have no illusions that they can hold a candle to today's DSLR's.
The big crutch I always hear is the print looks good at proper viewing distances. What is the proper viewing distance. Walk into any gallery and see people looking closely, deeply into the photos, looking at the details. This crutch we call proper viewing distance is just that, a crutch.