Am I missing something obvious, or is this seemingly simple thing impossible?

Why when I draw a simple shape, say a 100mm square, and then print it, the printed version is never 100mm square? The printed image is usually bigger than the dimensions I have set on screen.

Thanks for your replies.

If you use the simple print command, it tries to place the whole drawing into the paper size, whatever scale that will be. If you want to print in scale use the print preview and set the scale there. Then stay in that mode and use "print" from there.