You must be sure not to dolly the camera in ANY of the views before importing the image. You can ensure all the cameras are at the same scale by opening a fresh scene or going to each view and running view>predefined>(front, side, top, etc)
As an alternative you can assign your reference images to a poly plane and scale it.
There are advantages and disadvantages to both methods (using the camera image planes or poly planes)
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675
Last edited by ctbram; 27-09-2012 at 06:31 PM.