In MacDraw II, to precisely rescale a scanned image, first turn Show Size on. When a scanned image is selected, show size displays the horizontal and vertical resolution of the scanned image in addition to the size of the bounding box. You see these values change as you resize the object by dragging one of its handles. For instance, a 72 dpi image can be resized smaller to create a 144 dpi image.
You cannot resize scanned images using the Rescale When Pasting feature of MacDraw II.