To scale your artwork, check the "Lock Aspect Ratio" box in the left-hand column of the "Place Your Artwork" page. This will make it so that the vertical and horizontal measurements increase or decrease together keeping your image proportional.
To stretch your artwork, uncheck the "Lock Aspect Ratio" box in the left-hand column of the "Place Your Artwork" page. This will allow you to stretch your image either vertically or horizontally, independent of the other.