Hi Mark: The simple answer is that digital files don't work in inches, but pixels. Therefore, the 5D image size of 4368 x 2912 pixels, divided by 240 pixels/ inch (dots per inch for the printer, but this is the simple answer), yields a print size of 4368/240 x 2912/240 or 18.2 x 12.13 inches. You can print bigger images, but to print at the same resolution (240ppi), you need to 'create' pixels to fill in the new spaces. This is call uprezzing, and there are a number of software programs, including Photoshop, that will allow you do that.
Now there are many people on this list who have much more specific knowledge about this, and I leave the floor to them...
Mike.