When you load a graphic, you can use the params width and height to change its size, by combining this with the offset option, you could draw any section you want.
Alternatively, you can use any graphic (the .png file itself) of any FlxSprite and use it to do another resized sprite, heres my example:
var clone:FlxSprite = new FlxSprite(this.x, this.y);
clone.loadGraphic(this.cachedGraphics, false, false, Std.int(this.width), Std.int(this.height * 0.5));
I believe is now just this.cache, have to update. Anyways, I use that to cut in half a sprite, dinamically, so give it a try!