I was wondering if there was a way that is better than using the Toolkit.getDefaultToolkit().getScreenSize().getHeight()/getWidth();
and then using that as the dime
You're after what's called "full-screen exclusive mode"
Have a look at Full-Screen Exclusive Mode for more details.
Understand though, it doesn't play well with Swing.
Under Mac OS X 10.7, you can access the full screen support it provides for applications using com.apple.eawt.Application.requestToggleFullScreen(Window)
.
Take a look at Fullscreen feature for Java Apps on OSX Lion for more details