Mara Jade needs to be played by an actress who is not only beautiful, but she must be convincing in the action scenes as well.
Red hair is definitely a plus, but a dye job can always take care of that requirement.
The actress must make us believe that she can be a loyal and effective servant of the Dark Side, but she also still has that glimmer of goodness in her as well. Finally, she's got to look good with Luke Skywalker, just in case they become an item. So no tall ladies.