The answer is, "you can't."
Welcome to the world of web-design.
The problem isn't platform or browser, it's screen resolution.
You have no control over the screen resolution a viewer will be using.
The average person uses 1024X768, but lots of people (especially professionals) don't.
This changes over time, a few years ago 800X600 was the most common, prior to that 640X480 (similar to a television).
I, for instance use 1680X1050. Not only is this a larger screen resolution, but it is also widescreen meaning it really won't look the way you intend on my monitor.
The trick is to figure out how to work with this issue.
There are a lot of options. Some people tile a background behind it, others use a solid color, some have the flash window open in a pop-up sized to its dimensions. The solution can even be as simple as centering the swf so it looks framed by the white space.
A good place to find info on preloaders and other Flash tutorials is:
http://www.flashkit.com
You can also download some here:
http://www.newgrounds.com/downloads/preloaders/
