Well, don’t everyone answer this question all at once, now…
I guess I’ll answer it myself, in case anyone else has the question and wants the solution.
There is no mechanism by which to tell WM to make a shape a certain size. However, you CAN tell it to make a render extent a certain size, and in a certain place. With some careful math, you can set a render extent that will give you a guide box onscreen that is the size you need. Then you can simply fit the shape to the box. Once finished with creating the shape, you can delete the render extent (if you so desire).
Now, if someone could just tell me why the grid lines and the render box lines are all an unchangeable light gray which blends very nicely into the terrain coloration…
Ah, well.
Eric Storm