Fix sized objects independent of marker size

Hi, we are implementing AR.. when marker is detected , we are displaying the object on screen, but when we are moving the camera in out from the marker, the marker size displayed in the screen changes and similarly the object size changes. we want the object size should remain same irrespective of the marker size displayed on the camera...i.e when marker is near to camera feed then the marker size is bigger and object size too, where as when we move camera away from marker the marker size reduces depending on the distance and so object displayed on the is smaller.. we want the object size should remain constant,, irrespective of the camera distance from the marker.. Any hints??? Thanks Atik