Sorry, I don't use the sketch designer a lot and I could only make a guess... I'm not at home and I don't have Poser and the manual in front of me so I can't verify myself, but can you render a sketch designer animation? If so maybe you can select single frame and set a different size of the window... I don't know if it will work, some time ago a member here suggested this trick to render a picture of only the wireframe of the scene, maybe it works in this case too?