You can resize structural floors. When you select the slab edge, a drag control appears at either end of it. Click and drag this control to extend or shorten the slab edge.
You can flip structural floors by using the flip controls that appear in a 3D view, or, if you are in a 2D view (such as section), you can right-click on the structural floor and select Flip About Horizontal Axis or Flip About Vertical Axis. See Controls and Shape Handles.