Its the size of the width of the screen, lol. It depends on how large the resolution someone is using it, but if you want it to take up the whole screen no matter what the size of the user's resolution is then you need to make a 'liquid' layout. That means that it resizes to adjust for the size of th e screen. Basically, you will be setting the width of the layout to '100%' so that it will take up 100% of the screen.
You use percents instead of fixed values so the layout takes up '90%' of the users page instead of a fixed 700px for example.
