I can calculate image size in php. I am still learning flex, so I have not done that. However I found this.
var img:Image = new Image();
img.source = "image.jpg"
Most flex based bitmaps are visual elements with width/height available as properties. E.G. Bitmap, BitmapImage, Image
Otherwise, get a reference to the bitmapData and then you can access the width/height properties. If this doesn't make any sense, provide more information.