in the render tab, click on the dimention in the top left hand corner. This will bring up the render size... Then click the "Render to excact Resolution" That will render it larger but in the same window. When the render is done, click on the picture with an arrow coming from it in the upper right hand corner and this will pop the render into a new window for you. While this isn't exactly what you are looking for it's as close as I have found so far :)