I'm assuming you are referring to a flat plane for use as a background curtain. If that is the case, find the height x width of your image in pixels, then scale the X & Y of your plane accordingly. Load the image and afterwards use the general scale for the prop to the desired background size.

Personally, I hate linking from another source. If the connection changes, the image is lost from the archive. Uploading is permanent; you can go back several years and still find the desired information.

Edit: taking a second look at that right image, the normals appear very wacky. Don't use the double sided prop.