To resize your stage at initialize you can use following code (i assume you use javafx-2?)
//stage.setResizable(true);
Screen screen = Screen.getPrimary();
Rectangle2D bounds = screen.getVisualBounds();
stage.setX(bounds.getMinX());
stage.setY(bounds.getMinY());
stage.setWidth(bounds.getWidth());
stage.setHeight(bounds.getHeight());
or you can use fullscreen mode:
stage.setFullScreen(true);