There's no built-in option for what you're describing, but it's easily achieved with scripting, either from within Flash or on the JavaScript side. I'll put a code snippet at the end of the answer that ought to get you off and running.
First, however, let me suggest what you probably ought to do instead. Think of what happens when a user comes along who (regardless of monitor size) doesn't have their browser fullscreen. If you implement as you describe, they'll see your site at an arbitrarily small size, depending on how their browser is sized, and quite possibly see something small enough to be illegible and/or unusable.
Now sure, you could deal with that by further implementing a minimum size below which the Flash stops scaling, but then you're starting to do a lot of second-guessing about how users will want to see your site. Instead, it behooves you to ask, what would a user with a small monitor (or a small browser window) expect the site to do? And the answer to that is, show the site full-size and add scrollbars. And for my money, that's what your site should do.
So in your HTML (or in Flash's HTML export settings) embed your SWF into a 1000x750 container. This will keep the SWF from scaling for anyone - on your monitor you'll see the unscaled site centered (or top-left aligned or whatever) within the window, and on smaller monitors you'll see the whole site unscaled, with browser scrollbars.
At the end of the day, you have to realize that even though Flash content scales inherently, most web sites are made up of text and images, which don't. Even though you can scale down at runtime, it's usually not what the user expects and it usually doesn't make the site look good.
Now as promised, here's a snippet of Flash AS3 code that ought to give you enough head start to do anything you like with scaling. This ought to give you the desired effect, if you've published your Flash into a container that takes up the entire browser window, with the scalemode set to "no scale".
// perform initial size check
checkStageSize();
// register to re-check when stage size changes
stage.addEventListener( Event.RESIZE, onResize, false, 0, true );
function onResize( e:Event ) {
checkStageSize();
}
// do the scaling here
function checkStageSize():void {
var wid:Number = stage.stageWidth;
var hi:Number = stage.stageHeight;
var needToScaleDown:Boolean = (wid < 550); // or some other test
if ( needToScaleDown ) {
var scale:Number = wid/550;
someContent.scaleX = someContent.scaleY = scale;
} else {
someContent.scaleX = someContent.scaleY = 1;
}
}
This supposes that "someContent" refers to a container holding everything you want to scale, with its registration point at the stage's origin (upper-left corner). There's no easy way to automatically scale the whole movie, so you'll want to place your content in a container movie clip and scale that instead.
Feel free to put the snippet above in a frame action on the stage, change "someContent" to refer to a container you can scale, and play around with it. But I really think you'll wind up deciding that it's more usable to just show the site unscaled.