You can get this functionality using the Custom Draw Trigger by adding the following code:
double size = 5.0;
int shapeIndex = getshapeindex("fs3d\\Processor\\Processor.3ds");
double scaleFactor = size * viewpointradius(view).value / 100.0;
if (viewprojectiontype(view).value == 1) // orthographic projection
scaleFactor = size * 10.0 / viewmagnification(view).value;
fglScale(scaleFactor, scaleFactor, scaleFactor);
drawobject(view, shapeIndex, 0);
That example code draws a Processor shape. You could adjust the getshapeindex() call to draw a different shape. For example, use "fs3d\\General\\Plane.3ds" to draw a plane.
You can adjust the size variable to make it draw bigger or smaller.
Attached is a sample model demonstrating this.

Phil BoBo
Sr. Manager, Software Development