It seems to take a certain level of concentration for Robin to sustain her copied body parts. That's why she doesn't usually make more then 100 at once, and why she can't fly for very long, it's too tiring for her to sustain the arms and fly at the same time. The amount of concentration it would take to form an actual full-body clone of Robin would be enormous, exponentially greater then just making 100 arms. Perhaps near the end of the series she may be able to focus enough to do it, but as of now, while it's theoretically possible, she hasn't got sufficient strength and Devil Fruit control to pull it off.