Add the following as a new script, under the "Materials" heading, and above "Main Process" (as you do with most scripts) 

class Scene_Map
  #--------------------------------------------------------------------------
  # * Call Menu Screen
  #--------------------------------------------------------------------------
  def call_menu
    Sound.play_ok
    SceneManager.call(Scene_End)
  end
end

class Scene_End
  #--------------------------------------------------------------------------
  # * Create Command Window
  #--------------------------------------------------------------------------
  def create_command_window
    @command_window = Window_GameEnd.new
    @command_window.set_handler(:save,     method(:command_save))
    @command_window.set_handler(:shutdown, method(:command_shutdown))
    @command_window.set_handler(:cancel,   method(:return_scene))
  end
  
  #--------------------------------------------------------------------------
  # * [Save] Command
  #--------------------------------------------------------------------------
  def command_save
    SceneManager.call(Scene_Save)
  end
end

class Window_GameEnd
  #--------------------------------------------------------------------------
  # * Create Command List
  #--------------------------------------------------------------------------
  def make_command_list
    add_command(Vocab::save,     :save)
    add_command(Vocab::shutdown, :shutdown)
    add_command(Vocab::cancel,   :cancel)
  end
end


To reduce your game's final file size further, you could remove a lot of the inventory/battle code (for example, Scene_Battle), but I found that this does not save very much space in the end. Additionally, your time is better-spent focusing on the actual game than getting caught up saving a couple KB in file size. Focus on what matters; spend your time where it counts: on making the game - for the player - as awesome as possible. Your players won't notice a couple extra KB in the final download.