You can resize your application automatically within your browser window. Use a Viewport:

Viewport vp = new Viewport();
vp.setLayout(new FitLayout());
FitData layoutData = new FitData(5,5,5,5);
vp.add(new ContentPanel(), layoutData);
RootPanel.get().add(vp);

You should also set FitLayout to fit the ContentPanel of your application within the browser window.