augmented reality cards work is that a camera (like the cameras on Nintendo's 3DS) sees a certain marker image at a certain size on screen, and then displays video from the camera along with a virtual object that matches that size. The closer the camera is to the card, the bigger it is on screen, and thus the bigger the object is displayed, as if it really exists in the on-screen world.
So what if you took one of the slightly-smaller-than-a-credit-card-sized images, blew it up, pixel for pixel, into a 22 foot long swimming-pool-sized poster, and then looked at it with the Nintendo 3DS? Answer: You'd get a three-story tall augmented reality Mii. See it in action after the break.