The Single Straight Wall method produces one straight wall element at a time.
With the Wall Tool selected, choose the Straight Wall Geometry method from the Info Box and draw the wall segment on the plan.
In both Floor Plan and 3D, you define the length of Wall segments by clicking at their endpoints.
Note: This CAD-like drawing method is the default method of drawing segments in ArchiCAD, but you can change it in Options > Work Environment > Mouse Constraints & Methods.
The moment you start drawing the Wall, a ghost contour appears and it follows the cursor until you click at the second endpoint.
Walls can be stretched using either menu commands or pet palette commands.
See Stretching Walls.