With good craft, large prints from "small" files is quite possible, look great and my experience will tell you to ignore the math & naysayers and simply go for it!
You need to start with a well chosen, well-crafted image using your best technique, carefully process it then you should be fine. Don't let the math get in the way that tells you otherwise.
That said I've created 24x36 prints from an iPhone image because the image quality was good enough and hung them in up in a museum and the curator was impressed when we told her what we used. A well-edited and well-selected images, even just a 6 mp file, will handle a two-page spread. That same little image with the continued technique of good craft in the process will look good on the wall as a 20x30 or even larger print.
Nearby where I live is a hotel of several hundred 30x40 prints that I created from mainly 10-12 megapixel files. I still take small files today that many would say couldn't be printed well...and then I print them both large and well.
Today, the prints are are even better from a few more megapixels and a little more technology and practice with my current images though mostly this is from my technique of overall craft of getting the best image, meticulous processing and selective editing.
With your Canon Mk. III, 40x60 prints using your best craft are possible and should be quite stunning to the vast majority of viewers. There will always be nit-pickers who tell you otherwise and tell you every technical flaw, but a technically perfect image on a print without emotion and soul is but an empty sheet of paper. So
I'd tell you take your best image, print it large and put it on the wall, look at it, enjoy it then go out and share your vision!
