Main class for HaxePunk with variable screen size for mobile, tablet, and desktop discussion: http://www.openfl.org/community/general-discussion/developing-multiple-devices-screen-sizes/

GameScene.hx

package scenes;

import com.haxepunk.Scene;

class GameScene extends Scene
{
 
  public function new()
  {
    super();
  }
   
  public override function begin()
  {
    
  }
  
  public override function update()
  {
    
    super.update();
  }

}

Main.hx

package;

import flash.Lib;
import flash.display.Sprite;
import com.haxepunk.Engine;

class Main extends Sprite
{
  
  public static var engine:Engine;

  public function new()
  {
    super();
    
    var _stageWidth:Int = Lib.current.stage.stageWidth;
    var _stageHeight:Int = Lib.current.stage.stageHeight;

    var baseGameWidth = 0;
    var baseGameHeight = 0;

    //Check to see if the device is tablet sized
    if(_stageWidth >= 1024 || _stageHeight >= 1024){
      baseGameWidth = 1024;
      baseGameHeight = 768;
    }else{
      baseGameWidth = 640;
      baseGameHeight = 480;
    }


    engine = new Engine(baseGameWidth,baseGameHeight,60); //Use the base Game Size
    addChild(engine);


    engine.scaleX = _stageWidth/baseGameWidth;
    engine.scaleY = _stageHeight/baseGameHeight;
    
    #if debug
    HXP.console.enable();
    #end

    HXP.scene = new scenes.GameScene();
  }

  public static function main() { Lib.current.addChild(new Main()); }
  
}

project.xml

<?xml version="1.0" encoding="utf-8"?>
<project>
	
	<meta title="MyGame" package="com.example.MyGame" version="0.0.1" company="My Company" />
	<app main="Main" path="Export" file="MyGame" />

	<window fps="30" background="0x333333" />
	<window width="1024" height="768" unless="mobile" />
	<window width="0" height="0" if="mobile" />
	<window fullscreen="true" if="mobile" />
	<window orientation="landscape" vsync="true" antialiasing="0" if="cpp" />
	
	<source path="Source" />
	
	<haxelib name="openfl" />
	<haxelib name="openfl-bitfive" if="html5" />
	<haxelib name="HaxePunk" />

	<assets path="assets/graphics" rename="graphics" include="*.png|*.jpg" />
	<assets path="assets/audio" rename="audio" include="*.mp3" if="flash" />
	<assets path="assets/audio" rename="audio" include="*.wav|*.ogg" unless="flash" />
	<assets path="assets/font" rename="font" include="*.ttf" />

	<icon path="assets/HaxePunk.svg" />
	
</project>