I really haven't felt like it's worth chiming in on this stupid concept of "you can't print larger than 15 inches at 20 megapixels" but it's honestly just so hysterical I can't help myself.
My coworkers and I have had 22 megapixel 5D3 images blown up to *literally* over 100 feet to cover the side of a stadium entrance, and images to cover decals across an entire double decker tour bus. They looked friggin' awesome, and they definitely had no issues being printed from 22 megapixel files. I make and sell 40x60 inch prints from my 1DX2 all the time, even from crops.
I don't know if anyone's ever told you this but... there are ways to increase the resolution of a file to print at 300 DPI no matter the size of the print that look perfectly fine.. And those skills were taught in literally the first intro class I ever took in my bachelor's degree in photojournalism. It's not impossible just because you put a 20 megapixel image into Shutterfly and it told you that it would only go to 15 inches