Well, to get the screen size, you would use

Toolkit.getDefaultToolkit().getScreenSize()

That being said, if you are using 1.4 or later, you should set your window
size by calling

setExtendedState(JFrame.MAXIMIZED_BOTH)

on your frame. This has the benefit of allowing for a taskbar or similar
device taking up space on the screen, which simply setting your preferred
size based on the screen size would not do.

Of course, you should verifiy that the toolkit for your system supports this
by calling

Toolkit.getDefaultToolkit().isFrameStateSupported(JFrame.MAXIMIZED_BOTH)

first, since if, for some reason, this operation is not supported, your
frame will not resize.

Dave