Methods vary depending on the orientation and complexity of the stretching shape.
A simple rectangle is stretched by the marquee.
BUT, if you have a complex shape that must be proportionally stretched, it IS NOT simple in Archicad. Like so many other things people think should be simple, including automatic quality rendering and skewing.
Here's how:
1: Copy the linework/fill/whatever into a new object -- 2D symbol -- and save the object. Specify: "Hotspots on Bounding Box" and "Fit size to 2D Symbol."
2: Place the object.
3: Stretch the object to the length you want.
4: Explode into primitives.
Dwight Atkinson