in your events when the button to kneel is pressed, use the playerbox.height and playerbox.width events to change it. In my case the invisible sprite is called player box, you also have set size and set scale so between all those you should have what you need. Here is a screenshot of the options:
<img src="http://dl.dropboxusercontent.com/u/10285716/ConstructSampleScreenshots/ChangeSize.png" border="0" />