ActionScript 3 :: How To Know Width / Height Of JPEG Before Loading

I am trying to dynamically load a progressive jpeg using actionscript 3. To do so, I have created a class called Progressiveloader that creates a URLStream and uses it to streamload the progressive jpeg bytes into a byteArray. Everytime the byteArray grows, I use a Loader to loadBytes the byteArray. This works, to some extent, because if I addChild the Loader, I am able to see the jpeg as it is streamed, but I am unable to access the Loader's content and most importantly, I cannot change the width and height of the Loader.

The cause of the problem is that until the Loader has completely loaded the jpg, meaning until it actually sees the end byte of the jpg, it does not know the width and height and it does not create a content DisplayObject to be associated with the Loader's content. My question is, would there be a way to actually know the width and height of the jpeg before it is loaded? I would believe this would be possible, because of the nature of a progressive jpeg, it is loaded to it's full size, but with less detail, so size should be known. Even when loading a normal jpeg in this way, the size is seen on screen, except the pixels which are not loaded yet are showing as gray.