AFAIK you can't, since as long as the window is not shown, it doesn't have a size, as it could be it in a layout, or other influences take effect only at time when it is shown.
However, I *think* if your intercept the show event, you might have valid size values in it, so it could spare you the opacity trick.
But you have to test to make sure the geometry of the windows is valid at the show event it self.