Hi Stu,

To change the length of the stair you have to select the flight. You hover over the flight with your mouse. When the flight becomes darker then you are on the flight selection. You have to click and the flight will then become even darker so that you can see it is selected. The property screen on the right side of your screen will then show the details of the flight. There you have the parameters for the length that you can change. 

Regards

Michel