How to make this?

I’ve asked this question before but didn’t quite get a good answer. So here goes again:

In this site:
they use a background that seems to be 1600*1200 big (or bigger?). When I resize my browser window, the background image doesn’t seem to scale with it.

What I want to know: how do you make something like this when you start a project? Do you need to make a stage that’s bigger than 1600*1200, and draw your background in the size you want? Or is there some sort of trick to do this?

(Perhaps detect screensize and dynamically scale a background movieclip or something?)

What do you guys and gals think?