First, you have to set your projects settings to not build
the HTML template dir. this is done by right clicking on the
project, going to properties, selecting Flex Compiler on the left,
then unchecking the "Generate HTML wrapper file" checkbox.
The goal is to use fscommand to build a fullscreen file with
content in the center. I have attached code with two application
tags, one commented out. first, build it as is. You'll see the
content is centered. Then, hit escape, you'll see that the size of
the window is less than 1024x768. If you load the other application
tag, you'll see that the content window is the right size, but the
fullscreen is aligned top left.