Well, you would like to have something functional and practical that matches the outfit at the same time.  Too long is cumbersome, the material can snag on objects and can be a liability if a villain manages to grab it.  Anything with shoulder pads just impedes movement and range of motion.  If she needs to rely on fighting skills, she has to maximize and focus her attacks/counterattacks as efficiently as possible.  Marvel has a tendency to forego capes altogether, but, then again, they have come out with some rather outrageous costumes.