Images which are enlarged beyond the tracking image should display as it does within Designer. However, if you move your phone’s camera view away from the tracking image, Grab and Go is enabled, which tries to fit the content within the size of your screen. Grab and go also resizes your content based on the size of your content in other scenes (for example, if on the first scene you move your phone away from the camera view, if you’ve got an object which is twice the size of your tracking image on another scene, grab and go will resize to accommodate that large image, even if the scene is not in view).
There isn’t a way to make the image itself full screen on tap when you create your experience in Designer; to add that functionality, you’d need to use Studio. To make an image full screen on tap, you’d best look at the UI Coordinate Systems, Controllers and States and Buttons and Interactivity articles.
Hope this helps you,