The sourceSize property can be set by assigning width and height subproperties. The following example will ensure that the image only allocates storage needed for the visual representation, irrespective of how large the source image is - i.e. it will fill the Image storage bounds to parent, not using the image source native resolution;

Image {
   anchors.fill: parent
   source: "images/reallyBigImage.jpg"
   sourceSize.width: width
   sourceSize.height: width
}