You're kidding, right?
I mean, that's the whole point of a mucho-megapickle camera.
Have a look at any of those 100% crops you see wherever. No, not the underexposed unfocussed high-ISO macro shots of the inside of a lens cap that were pushed by ten stops in post-processing -- I mean real shots made with good glass and good technique. Preferably after a competent artist has done all the requisite post-processing needed with any image to really make it shine.
Notice just how amazingly good that 100% crop looks?
Well, it represents an actual-sized crop from a print even larger than what you're asking about.
The 5DIII can easily do 36" x 54" prints -- assuming, of course, good technique at all steps. If you know what you're doing, 44" x 66" prints shouldn't be a problem, but the assumption there should be that viewers won't be getting much closer than arm's length. Stick your nose in the print and it'll be a bit on the soft side, but not objectionably so. Bigger than that and you're no longer talking about prints; you're now in mural / billboard territory, and even the original Digital Rebel (300D) is capable of cutting the mustard.
Unless you're cropping, there's really no real-world practical need for more resolution in a 135-format camera than what you get in the 5DIII. If you actually need more -- and damned few people do, no matter how they blather on on Internet forums -- then no 135-format camera will ever be adequate; at a minimum, you need medium format. And, if that's what you need, you can easily afford it.
After all, if you really are selling 4' x 6' stick-your-nose-in-it landscape prints, you're selling each print for at least as much as a 5DIII body, if not more.