do you mean: the card should match the image aspect ratio of you Mattepainting?
that should do it by default if you plugin your image in the card (and on the card node the “image aspect” checkbox is on)
because the size of a card is nothing real, the “scanline renderer” and the camera you use define the size of the card in the final picture
maybe you want to have the card directly in front of your camera and view always the full image?
there is a nice little trick:
copy/link the transform and rotation of the card from the camera
copy/link the “lens in focal” and “lens in haperature” from the camera
play now with the “z” slider on the card to position you card in 3d space in front of the camera