Make sure that "Keep Proportions" in the Render Room is unchecked. Then, in the Assembly room, put your cursor over one of the corners of the production frame and drag to resize. If you cannot see the corners of the frame then zoom the window out with "Z-Alt"(PC) and make the frame smaller, then Zoom back to 100%. Keep the imaged centered while zooming or your camera perspective may get messed up. HTH