That depends slightly, because there are many ways to add the background. In some themes, we have set the background to simply scale with the stage. See the
persimmon and
zanzibar themes.
As you can see with these themes when you resize the browser window, the background will scale proportionally with the stage size. This works bet with SWF background with vector graphics though, as scaling will have no negative effect on the graphic.
In other themes, the background is locked and will align to either middle or a corner. If you have an animated SWF background, I would perhaps consider setting it to scale to the background. If you are using a bitmap background, you should perhaps not set it to scale. The size of your background should perhaps depend on the background content - As you can see with the default theme, it is a simple diagonal gradient that is aligned to top left. If you have a large background, you may want to align it to the middle, and simply make it "big enough" ... Or you could make sure it fades into the solid background color.
As you understand, there are many approaches to this. I would need to know your general background image to give a recommendation.