this code works

package
{
          import flash.display.Loader;
          import flash.display.LoaderInfo;
          import flash.display.Sprite;
          import flash.events.Event;
          import flash.events.MouseEvent;
          import flash.net.FileFilter;
          import flash.net.FileReference;
          import flash.text.TextField;
          import flash.text.TextFieldAutoSize;
 
          public class Main extends Sprite
          {
                    private var fileRef:FileReference;
 
                    public function Main():void
                    {
                              if (stage)
                              {
                                        init();
                              }
                              else
                              {
                                        addEventListener(Event.ADDED_TO_STAGE, init);
                              }
                    }
 
                    public function init(e:Event = null):void
                    {
                              removeEventListener(Event.ADDED_TO_STAGE, init);
 
                              fileRef = new FileReference();
                              stage.addEventListener(MouseEvent.MOUSE_DOWN, onDown);
 
                              var txt:TextField = new TextField();
                              addChild(txt);
                              txt.autoSize = TextFieldAutoSize.LEFT;
                              txt.x = txt.y = 20;
                              txt.text = "click anywhere to load an image file...";
                    }
 
                    private function onDown(evt:MouseEvent):void{
                              fileRef.browse([new FileFilter("Images", "*.jpg;*.gif;*.png")]);
                              fileRef.addEventListener(Event.SELECT, onSelected);
                              stage.removeEventListener(MouseEvent.MOUSE_DOWN, onDown);
                    }
 
                    private function onSelected(evt:Event):void{
                              fileRef.addEventListener(Event.COMPLETE, onLoaded);
                              fileRef.load();
                              fileRef.removeEventListener(Event.SELECT, onSelected);
                    }
 
                    private function onLoaded(evt:Event):void{
                              var loader:Loader = new Loader();
                              loader.contentLoaderInfo.addEventListener(Event.COMPLETE, onComplete);
                              loader.loadBytes(evt.target.data);
                              addChild(loader);
                              fileRef.removeEventListener(Event.COMPLETE, onLoaded);
                    }
 
                    private function onComplete(e:Event):void 
                    {
                              var loaderInfo:LoaderInfo = e.target as LoaderInfo;
                              loaderInfo.removeEventListener(Event.COMPLETE, onComplete);
 
                              var loader:Loader = loaderInfo.loader;
                              trace("loader size: ", loader.width, loader.height);
                    }
          }
}
 

sorry i initially left a bit off and have since updated the code.

if you try and get the width and height of loader just after you call loadBytes you will get 0, 0

you have to setup and eventlistener for Event.COMPLETE and when you receive that you can access the width and height

no worries

the file reference loads the bytes into a ByteArray. Loader then needs to load the bytes from the ByteArray. It will be pretty quick but you still have to wait