The new window actually can be bigger than the workspace. As joffry mentions you should set your window size to the same aspect ratio as the workspace window, then you won't lose any details. However Poser won't render in a new window larger than the work space window. Instead it renders the whole picture in a sort of "keyhole view" and you will only see the rest of it by using the scrollbars at top and bottom of the new window when the render finishes. The picture will save just fine, at full size. If your using P4 the maximium render size can be about 4000 pixels square, although you can do bigger as an animation.